I envisioned it as a manga-styled (probably not literally right-to-left, but art-style, plot, the everything, stereotypical shonen manga) story, and named it 'SirPlus' (because while I imagine that pun is known, it's not something I think would be an actual story's name, whereas Surplus, the title that I'd otherwise want, would almost certainly be taken), after the titular character's eventual codename.
So to describe this story, I need to give some background info. The main inspiration is YuYuHakusho, with a significant dash of InuYasha but with sides of various other series which feature things like Ki or an equivalently-named body energy source.
The idea spawned from wanting to run a series featuring five energy types--and this being a stereotypical shonen series, yes, the protagonist can, eventually, tap into all five of the energy sources (and his codename derives from his ability to do so, thanks to a commonly-used reference chart for the nature of the five energy types).
The five energy types are as follows:
Holy Energy, considered the opposite of Demon Energy, has a few requirements. The user must be pure of heart (but mind you, 'pure of heart' has a very very loose definition; a chivalrous pervert can still be considered pure; someone who is pure evil is still pure; a well-intentioned extremist with pure intentions counts, and so on and so forth).
You must learn how to use Holy Energy while you are currently alive. (Mind you, you can use it if you've learned it even if you are dead; if you were a spirit who was through any means given a 'living' body, you can then learn it while using this body even if said form is only temporary.)
You cannot be a pureblooded demon; no demon can ever learn holy magic. Heck, while means of becoming a demon (though rare) may exist, if someone knows how to use Holy Energy, they cannot become a pureblooded demon. To become one, they'd have to give up their Holy Energy capacity altogether.
And finally, you must have training in how to use Energy--usually, this is easiest done if the energy you are training in is Spirit energy, but Body energy is just as commonly used (mostly because Body energy can be learned by literally anyone with training whereas Spirit energy has some requirements of its own).
With the exception of Kekkaishi, Shrine Maidens, and Holy Monks (each of which is a special exception because they use a specific variant of holy energy), this energy type is not inherited genetically. You can be born the child of a Holy Energy user and be unable to use it; you can be born the child of a family who nowhere in their family has anyone ever used Holy Energy and yet have the capacity to tap into it; you can be surrounded by Holy Energy users and never learn it yourself; you can have never ever ever encountered a Holy Energy user in your entire life and yet still pick up how to use it.
Literally any non-demon can use it, so long as they meet the specified criteria; pure of heart, currently alive, not a pureblooded demon, and trained in how to use Energy (so basically, Holy Energy is almost never the first of the five energy types learned unless you're a Kekkaishi, Shrine Maiden, or Holy Monk).
However, in spite of this capacity of anyone being able to learn it, it is the least-learned type of energy out there of the standard four. Its primary users are the Kekkaishi, Shrine Maidens, and Holy Monks, who learn it as part of their family legacies. (You don't need to be born into one of those three to take it up, but it is certainly much easier if you are born into a family of one of those three. You also aren't required to use that specific form of Holy Energy. Someone who is the child of a Kekkaishi can learn customized Holy Energy magic which doesn't involve Kekkaishi barriers at all.)
Kekkaishi, Barrier Warriors (who have an approximately equal number of active female and active male members), are mostly defensive. Their barriers can purify evil within them (which also gives them minor healing abilities, a frequent attribute of Holy Energy), but otherwise it takes creativity to use them offensively. Creating small spear-like barriers; crushing someone between two barriers; creating a barrier and removing the oxygen from within to asphyxiate those within; creating a barrier and exploding something within to create a contained localized explosion, etc.
Shrine Maidens (who are mostly, but not exclusively, female...though they are still called Shrine Maidens even when male and wear the Shrine Maiden clothing regardless of gender) are mostly offensive. They have limited healing abilities and are very skilled at detecting evil, finding weak points in enemy's defenses, and offensive usage of magic, mostly through either enchanted physical arrows (these are more powerful) or outright pure energy arrows (these are less powerful but have unlimited ammo and don't require lugging around a physical bow/quiver).
Our protagonist's mother is a semi-retired shrine maiden. She no longer travels across the country (something active shrine maidens are expected to do), but she still helps out at her shrine, around town, and trains the next generation of shrine maidens. Our protagonist refused to take up the family tradition, so isn't trained in shrine maiden holy magic, but he is trained by her in how to fight, skills he'd later transition into his own style. (He ends up using customized Holy Energy eventually, although it is not the first Energy type he learns even though he's obviously known about it his whole life.)
Holy Monks (who are mostly, but not exclusively, male) use a balance of offensive and defensive applications of their holy magic. They use charms, they use enchanted staves, they use holy fists/elbows/knees/feet, can launch some energy attacks, ward against attacks especially from evil, purify evil from an area, etc.
All three have plentiful membership and all three have similar tendencies. Some stay local, to help out around the town, whereas others roam around the country (even world), helping out others, and it is expected for them to protect, to heal, to ward against evil and so on and so forth.
All three also happen to have been taught and learned that while a fair share of their enemies would be demons, demons are not inherently evil nor are all evils in the world originating from demons and are quite aware of what their fellow human beings are capable of, both the good and the bad.
But speaking of demons...
Demon Energy, on the opposite side of the spectrum from Holy Energy, is innate to anyone with demon blood in their veins. It is also exclusive to them. There's not much more to say than that. Only demons can use it, and every demon inherently uses it. Obviously, some demons use it more than others, but every demon has the capacity to use it to at least some degree.
Spirit Energy has similarly complex requirements to Holy energy. The user must have died--but mind you. This death doesn't require you to currently be dead. Clinical death counts. And, yes. That loophole you probably just thought of? It is exploited. If you are dead for less than a minute, it still counts as a death even if you are immediately revived.
Because this aspect of spirit energy is known and well-documented, it is commonly exploited in fact. Obviously, unlocking it via this method is considered dangerous (there's always the risk the revival fails), but it is still an oft-used albeit slightly dangerous method of unlocking this energy type for use.
In extremely rare cases, near-death experiences (especially accompanied by some combination of: latent untapped body energy potential, extremely strong presence of Spirit Energy in the area over a sufficiently long period of time, a brief comatose state accompanying the near-death experiences) can unlock it.
Spirit Energy also takes training to unlock and master. One need not unlock another type of energy first to use it, but it's certainly easier to learn spirit energy if you know how to use a different type of energy first. It is also much much easier to learn if you are a spirit, especially in a currently-not-in-a-living-body state.
Mind you--spirits who have obtained a living body don't magically lose access to their spirit energy; once unlocked, it's unlocked permanently even if you're well and truly fully alive the entire time.
Life Energy, on the opposite side of the spectrum from Spirit Energy, is never used by anyone even though almost anyone can use it. The 'almost' comes from it having a pretty simple restriction: its user must currently be alive, thus, spirits can't use it (and thus why it's considered to be on the opposite side of the spectrum).
Only a spirit sacrificing a revival method can use it without consequence; only a spirit with a currently-living vessel can use it, but doing so drains the life of the vessel. Life Energy is, simply put, sacrificing your lifespan. All beings, even demons who have lived for millenia, have a lifespan, and using life energy shortens it. It is irreversible; it is not something which you can just go use and then try to replace by using a different energy method (say, a demon trying to use a demon power to maintain their immortality).
Once gone, it is gone. It will shorten the time you have alive, permanently. It is finite in its supply. It is a measure of last resort, to be used only when you are ready to give up your life in a last-ditch effort to well and truly fight to the bitter end in defense of your objective. It cannot be forcefully used. It cannot be coerced out of someone. If you were, say, controlling the body of someone you wouldn't be able to kill them by making them use life energy. It requires conscious, deliberate, fully willing, thought to use as a measure of last resort, where you have pushed yourself to your limit and need a final push to actually accomplish the job.
As a consequence, unlike other energy types, it usually cannot be drained from others. (Energy drainers are in of themselves fairly rare, but they exist. Yet none of them can drain life energy. Well, almost none of them...) There's a minor exception; an antihero character (might be a rival to the protagonist? Not sure yet about that) can drain it...but only from those who consent to the drain. He tells them that he can fight, he can save them, but that they need to, say, give up one year of their life to do so and asks them if they accept; only if they do does he have the ability to use their life energy in the amount quantified.
There'd also be one vampiresque villain biting the neck of a victim with the ability to drain it--but only if the victim has absolutely no defense, no way of fending off the attack. No training, no resistance, no awareness, and no incredibly strong latent energy of any type.
The protagonist only uses it once, but it was a very notable instance.
The final type of energy, Body Energy, also called Ki Energy, is something literally anyone can learn to use. Everyone has it; everyone can use it. There's no restrictions; there's no limitations. All it takes is to train in it, or to be innately gifted with it and exposed to a sufficiently large trauma as to forcefully unlock it. (And for obvious reasons, the former is far more common than the latter and is the preferred method.)
None of these energy types are limited in what they can do; the only limits to them are the aforementioned requirements to be able to use them in the first place. They can do literally anything the user wants them to, provided the user has both the sufficient reserves of the energy (again, none of these energy types exist in unlimited quantities for a person; everyone has a finite supply at any given time) and the sufficient training in mastering what they want to do with it. (If you want to launch a fireball, you can't just spontaneously think 'launch a fireball' and it'll happen; every technique you use is something you need to train your body how to learn how to use.)
Now, there is a bit of a 'trump triangle' to energy types. Holy will, usually, be able to overcome Demon energy. Demon energy will, usually, be able to overcome Spirit energy. Spirit energy will, usually, be able to overcome Holy energy. But these aren't hard rules so much as general guidelines of "all other factors equal, one energy type has a slight advantage over the others".
For what it's worth, Ki is mostly neutral to all three but could potentially overall be at a slight disadvantage; Life energy is mostly going to have the advantage over all three but harvesting it is not going to guarantee victory.
Now it is worth mentioning: you know how in basically every single anime show ever, each and every single person has different moves and techniques? How they are customized to the individual; some series may have some standardized techniques and some series may have characters who trained together using at least one of the same moves while having personalized unique individual moves to not be completely identical; same principle applies here.
Energy, regardless of type, is highly customized to the individual. Each and every individual learns their own techniques. They name their own moves, they discover the motions for their moves on their own, they learn how to channel the energy and how to shape it to do the desired effect, all on their own.
They can also be taught by another how to do it, by being trained by them. Seeing it done, going through the motions of it being done, and so on and so forth until they learn how to accomplish the technique.
Thus, while there are standards, while some abilities (e.g. Kekkaishi/Shrine Maiden/Holy Monk ones) are standardized, most powers are unique to the individual. And even said standard abilities? Can be modified/customized by an individual, quite often, to somehow mold them differently from their original form/shape.
So these energies both simultaneously are and are not "magical do anything" energy types. You can do anything that you have trained yourself to be able to do. Shoot a spear of energy is something that if you trained how to do you can do, but if you wanted to spontaneously shoot a spear of energy, good luck. Create an energy shield is something that if you trained how to do you can do, but if you desperately needed to defend against an incoming attack and shouted 'shield!' without having actually trained yourself in how to create a shield...good luck.
This training isn't just mental, nor is it just physical; it is a combination of both. You need to actually practice, you need to actually learn, you need to actually figure out how something works, in order to do it. And skills don't instantly transfer from one energy type to another (with the exception of life energy which would never be used if it didn't have this ability setting it apart from the other four; nobody's going to willingly burn off their lifespan to train in how to use it, obv). You can't practice a skill using demon energy and then try to use that same skill using spirit energy; to pull off that skill using spirit energy, you need to practice using it with spirit energy and figure out the difference.
Mind you. Once you learn how to use one energy type, it's usually not that hard to transfer this knowledge into applying a different energy type. If it took you ten years to learn Ki, it might take you only ten weeks to figure out how to use spirit energy techniques that are similar to your body energy ones. But it is not an instant "it's active, therefore I can transfer one to another".
The main advantages to having access to multiple types of energy are as follows:
-As mentioned, certain energy types can have the edge over other energy types, situationally. So knowing how to use multiple types allows you to respond in the way most likely to give you the advantage.
-Certain energy types are more easily suited for certain skills than others. While every energy type can do what the others can, certain energy types have easier times doing specific tasks (say, forming an energy blade) than others do.
-The body only stores a finite amount of every given energy type at a given time. You don't have unlimited body, demon, holy, spirit, or life energy; they are a resource that is depleted and needs to be restored with rest and recovery.
...However. It's not "your body only stores a certain finite amount of total energy"; it's "your body only stores a certain finite amount of each energy type".
So basically, having access to two energy types doubles your energy pool; three, triple. Four, quadruple. All five, you can fight until you drop dead from expending your life energy which would presumably be the last to be fully depleted.
I wanted to make a story based around that concept.
What I came up with was that the protagonist was the son of the aforementioned Shrine Maiden and, secretly, unknown to him until much much much later in the story, the demon Rakan. (His father's actively involved in his life. Has a part-time job, is involved in housework, and the like. So the reason it's not known isn't because his dad's not around; it's just that his parents never told him that he's half-demon and never told him about his demon lineage so while he knows about his mother being a semi-retired shrine maiden, he has no clue that his father's a semi-retired monster.)
Rakan is...well, the pioneer of the Demon Bullet and Demon Blade techniques (the protagonist's first techniques were ki-based versions of these techniques); for centuries, he's been known for his rampages where he slaughters and literally devours humans.
He derives no pleasure from consuming human flesh, nor does he need to in order to survive. (He can live just fine off of normal food.) However, whenever he consumes human flesh, he gains a temporary huge boost to his power...and 1/100th of this boost is permanent so if he were to consume 100 humans he would effectively permanently boost his power as much as he would temporarily from a single serving.
He does like to fight opponents who put up a fight. Mind you, 'putting up a fight' does not translate to 'can actually threaten him'; usually when he went on his rampages he'd find the people taking up arms against him and shred them to pieces slowly. Enjoying the hunt, in a sense. And if someone tried to just...not fight him, he'd find a way to incentivize them into doing so.
In his own words he fully admits that while demons are not inherently evil, he is. (Well, mostly. He's under no obligation to his wife not to commit such acts. He never promised not to, he was never asked not to, she never instructed him not to, but out of love just chose to not do so anymore. Not because it'd be preying on her kind. Just because it seemed like a romantic gesture to him. He's got no intention to start eating humans again, but he didn't change his morals or anything of the sort. In his own words, "I fell in love with her, not the human race she belongs to.")
It'd have been fully possible for him to increase his power by nonlethal methods: consuming recently deceased corpses (they need to be fairly fresh for him to get the power), or even taking nonlethal bites out of a person (which even with the best of healing abilities will leave a scar, but which is easy enough for him to do). He killed humans because he thought it fun to do.
This earned him a lot of titles.
Rakan the Devourer.
Blood Bullet Rakan.
Demon Blade Rakan.
Lord Rakan. (The title demons, particularly those who consider themselves his servants, call him.)
Ninth Hell Rakan. (Basically, the ninth demon to be considered so powerful as to be hell on earth.)
Rakan the Slasher.
The Striped Tiger.
Crescent Heavenkiller Rakan.
He insists "most of those titles I earned by killing some dude who had a few of his own"--
Said person those titles were earned by killing? A demon by the name of Ivan the Unkillable, AKA, The Unslayable Serpent, AKA, DragonGod Ivan, AKA The Hand of Heaven. (He was later revived and thus, is still actually alive. Still, the act of killing him was enough to be worthy of attention.)
Ivan was a demon considered powerful enough where if he were so inclined he could've been powerful enough to be a Hell--and only wasn't one because he was actually fairly saintly in that he was a genuinely altruistic demon with no ulterior motive, a down-to-earth demon who was friendly and amicable...just, immensely powerful. (And is half the reason why people know demons aren't inherently evil even though quite a lot of demons are.)
Rakan insists that the only reason he was able to pull it off was that he had just fed on an entire city's worth of people and thus was at the absolute highest possible peak of his power, and in his own words, "I thought he was just some schmuck who objected to me slaying a city; if I had known who he was, I would've run."
Rakan certainly didn't help his reputation when, after the aforementioned event, there was a conference held in the afterlife on whether to hunt him down and kill him due to him being such a large threat. Rakan killed himself (after setting up a revival mechanism so that he'd be brought back to life) in order to invite himself to the conference and offer his viewpoint explaining the matter.
This would basically be akin to party crashing in on heaven when you're Lucifer and saying, "Yo, God, sorry about the mess I caused downstairs, but it was an accident, I swear; I came here to apologize for the misunderstanding." He said that the city slaying was done as an experiment, he found it boring, he promised he would never do something on that scale again, explained he had no intention of challenging people like Ivan, and promised to do them a favor so long as they didn't put out the kill order on him.
He got what he wanted; they decided not to put out the kill order on him...by instantly on the spot classifying him as the Ninth Hell--threats so dangerous that nobody is to try and kill because they are just that dangerous. (He later paid off that favor simply enough; they asked for him to be present in a place, which convinced one of the other nine hells to negotiate rather than fight as they were originally going to until Rakan was there.)
He also said that he would mitigate the damage he'd do by continuing to do what he was already doing: hunting down demons (particularly evil ones) of notable strength and killing them, so that they wouldn't menace people. He had ulterior motives for this, of course; not done out of any sense of altruism, but simply done to seek a challenge, in that he wanted to have some good fights against opponents he knew he could beat but which he'd still have some difficulty in bringing down, where it wouldn't be easy to massacre them.
So he is, as far as demons go...rather noteworthy, rather notorious, both by spirits and by other demons because of his actions over the course of centuries of carnage and mayhem.
The protagonist inherited his father's powers, albeit dormant. As he is half-human, his demon powers passively grow stronger with time without him needing to consume human flesh. (He can consume human flesh to gain that immediate power spike and 1/100th of said power spike permanently being added...but as he's no desire to partake in cannibalism, for obvious reasons, opts not to.)
There's a tradeoff, though. Our protagonist is a really big eater who needs to eat a ton of calories to fuel said demon powers. In fact, when activating his demon powers, he consumes his own body fat and looks unnaturally muscular as a consequence when using his demon powers. He has the power to surpass his father quite easily, it's just that in his current state he can only use his demon powers at their fullest for a limited amount of time as a consequence of currently requiring that consumption of his own body to activate them.
Mind you, that is not a normal demon trait; it is a trait specific to "newblood" demons, where their demonic powers can only be activated by a trigger mechanism of some sort. His trigger mechanism just so happens to be his father's trait of consuming flesh. Given enough time to develop, he'd be able to use his demon powers without the need to consume his own body flesh, and consuming his own body flesh would be an optional thing done to give his demon powers an immediate power boost.
I guess I should talk about the world the setting takes place in.
In this setting, demons have faded in and out of common knowledge. Sometimes, they're out in the open and everyone knows about them; other times, they're in the shadows and very few humans are "in the know". In the current modern era the story is set in, people are slowly transitioning from a long period of the former into the latter; demons aren't completely out in the open, but they're less and less secretive.
There's not an unlimited number of afterlives, but there is a very large number of afterlives for dead people to go to; they will go to the afterlife most suited for them. Some could be described as 'hell', most would be described as some form of 'heaven'. There's a transitionary realm between our world and said afterlives, where there's a collective bureaucracy of spirits who help make the transition of spirits into the afterlives easier.
The existence of spirits is not public knowledge, but it's a bit of an "open secret"; basically everyone who uses energy or is closely tied to someone who uses energy has some awareness of spirits' existence. They are, after all, heavily involved with the world.
Usage of ki is something everyone knows about, though. Everyone knows how ki works; everyone knows ki exists; everyone knows what ki does, and so on and so forth. Not everyone learns how to use it; in fact, most people don't. But it's a thing that everyone knows about.
Martial artists are a big thing, as are organizations that travel across the world. Some official, some unofficial, some international, some by country. Militaries and Police Forces usually have many members trained in the usage of at least one form of energy; Detectives with powers investigating usage of powers are a thing; etc.
The setting would, obviously. Have Loads And Loads of Characters. Some muggle, most not; some who start muggle and gain powers, most who were already using energy of one form or another at the beginning of the story. (Our protagonist starts off not having any, but very very very quickly learns ki, as the need arises.)
There's probably more details to the story than these, but this is most of where the idea came from.
It's a big project and you can maybe see why I wanted to make it. I really love the idea.