Some of this wasn't new, but a lot of it was. I already knew he was a master swordsmen, but to what extent I hadn't laid out--ultimately I decided on "absolute best" as the answer for him, with him having no equals or superiors in terms of swordplay. His style is pretty similar to the protagonist's:
He's left-handed, but deceives opponents into thinking he's right-handed. His primary weapon in his left hand is a dagger/shortsword (would need to look up what name would be best--it might be a wakizashi, might be shorter than that, or maybe slightly longer, not sure exactly), known as the black wing: a black blade.
His secondary weapon in his right hand is a nodachi (longer katana), known as the white fang. Against most opponents, similar to The Whirlwind, he uses the black wing as his first weapon, but in a way which makes it clear he is not fighting at 100%: his white fang being sheathed, him using it mostly for defense, him basically clearly holding back at every stage, especially since he broadcasts his ki and lets opponents know how much he is using (not much)--this ki in his black wing is also defensive ki, another tipoff to him not fighting seriously.
When he is fighting an opponent of reasonable skill, he unsheathes the white fang and starts to use it, giving it the appearance of being going in for the killing blow: he mainly uses his black wing to trap an opponent, then uses his white fang to strike a killing blow. (Or, at least, decisive blow.)
...But when he is truly fighting serious, similar to The Whirlwind, it becomes clear that the simple technique is actually more deadly than it appears. This is accomplished because he is a master of defense-ki and attack-ki, aided by his blades with a slight touch of trick-ki.
He uses an initial assault with both blades (still using defense ki), then sets his nodachi up to deliver a blow--in this moment, he instead traps the opponent using the nodachi, switches to an attack ki, and uses the black wing to strike at the opening. This largely works as a deception because over the course of a battle, his opponent will have adjusted to the idea that the right side is the main threat, whereas it's really the left. The left is deliberately underplayed, as to appear weaker than it is, but it's much faster than the slow, awkward swing of the long blade, so it strikes in an instant.
Basically, the strategy is to set it up so opponents think that the smaller blade is meant for defense and redirection only, and that the larger blade is where the damage is really dealt from...but it's not; the larger blade is the distraction, to set up the smaller blade's blow. Only one opponent he ever used his finisher on lived to tell the tale; all others never got that far (not seen as a large enough threat for him to fight full-out) or died as a result.
In addition to being a master swordsman (one who inspired The Whirlwind greatly), he's a good judge of character, a charismatic and inspiring leader, a strategic mastermind on the battlefield with a good head for tactics and specializing in logistics in particular; he understands the battlefield and knows how to handle a kingdom, basically, the perfect ruler, a badass fighter who knows how to treat battles and peace alike.
While not perfect in all aspects, he has it where it counts, with a style suited for him: no perception ki, but perception ki isn't really something he needs because he doesn't need to sense which fights he can't win. No masking ki, because broadcasting his strength is actually beneficial to his reputation; any opponent WITH perception ki looking at him would instantly get a nonverbal, "Don't. even. bother.".
His trick-ki isn't exactly high; it's mostly a defensive ki and an offensive ki he has specialties in. And there, they manifest rather specifically: his swords lack guards. Both go straight from the hilt into the blade. That's one of the reasons he relies on a defensive ki: to prevent losing a digit or five when wielding his weapons. The defensive ki basically serves to deflect blows away from him. The offensive ki serves to extend speed/strength/range of strikes.
On that note, I developed a dodecagon of stats, ranged 0-100%, which could be applied to the series. As in, every character could have as a miscellaneous extra a "data sheet" where these stats would be referenced. (Something done on things like, say, Bleach. I think Naruto also had something like that, too.) Where there's a clearly defined minimum, no defined maximum, and stats aren't necessarily "rules" which dictate the victor of a fight before it has begun (especially since these stats could change over time or in varying circumstances).
Just something for fun, as an approximation. These stats would be the following:
Attack Ki: Already talked about, but basically: at 0, entirely lacking attack ki, attacks by a person are not enhanced. They're entirely normal. A blade is as strong and as durable as realistic: they will shatter, they will break, they will bend, they will not be able to cut through rock or water or air or the like.
At 100, attacks by a person are faster than should be possible, pack more of a punch than the mass/speed of the weapon should carry, blades slice much deeper, can cut through things they shouldn't, will never bend nor break, and can even extend slightly beyond the weapon, hitting a target even without making direct contact, usually cutting them.
The average for attack ki for those who possess it (basically, those above 0) is 30.
Defense Ki: Also talked about: at 0, entirely lacking defense ki, everything makes contact exactly as it should: no supernatural resistance causing attacks to somehow miss, no supernatural durability, things break and get cut and bend exactly as they should.
At 100, a person can block attacks (even enhanced by ki) to no ill effect: think, catching a sword barehanded, deflecting a blade with nothing but a forearm, a sword being able to withstand anything, the like. The average for defense ki (above 0) is 20.
Perception Ki: At 0, a person is entirely unable to detect ki at all. They may know ki exists, and may even be a user of ki, but they can't visibly see ki, nor can they determine when it's being used. Users of ki can figure it out easily enough, but they don't actually have the ability to innately sense it; they don't have the ability to see what type of ki is being used, they can't determine how much ki is being used, they can't determine the effectiveness of the ki, the efficiency of the ki, they can't determine skill of a ki user, except by using educated guesses, because a skill of 0 is blindness towards ki. A surprisingly large number of ki users have this, but it's not much of a hindrance, because ki users, being ki users, are usually experienced combatants and with the experience of fighting they're usually pretty smart in order to stay alive.
At 100, a person is able to almost always sense the ki of another, and know exactly what they are using. They can almost always pierce through even the best trick ki, though it's only 50/50 on an equal 100-masking-ki. They basically can always determine what's happening in terms of ki, and often in terms of the battlefield in general. They see things, they understand people well, they are practically mind-readers. They can anticipation motion. They can, faster than anyone else, figure out exactly what an opponent can and will do.
The average for perception ki (above 0) is 60.
Masking Ki: At 0, a person is entirely transparent: everything about them is open to everyone. Their movements are easy to read, they can't hide that they are a ki user, they can't hide their skill at using ki, they can't hide their presence, they are easy to see at all times. (Note this is not necessarily a bad thing.)
At 100, a person is in entire control of their presentation. They have complete mastery of what they can, and cannot, show off. They will give what they want, and not give what they don't want to give. They're invisible if they wish to be, or deceptively small, they are impossible to read, they are not what they appear to be.
The average for masking ki (above 0) is 30.
Trick Ki: At 0, a person's ki is always invisible, and always unable to directly alter perceptions or cause any unusual effects in general. With none, a person is not able to deceive a person into thinking something has happened which has not. (They can make a wrong assumption, sure, and a person can make something happen and make it appear it has not with good masking ki, but nothing happening yet appearing to have something happen is trick ki.)
At 100, a person's ki is basically tangible and visible, even colorful: basically, a Battle Aura. If infused with other types of ki, it can, say, cause a sword to spontaneously combust and have that actually be real because the ki is imbued within the object and rapidly heats it up. At this level, to a large extent, Your Mind Makes It Real. Trick ki is very diverse in what it can do: make something appear to happen which doesn't, make something actually happen which should be impossible, hide that something is happening, make something happening appear to be something else that is happening, all sorts of things, within reason.
The average for trick ki (above 0) is 40.
Ki Mastery: Basically, this is a sort-of "mana" stat: this is a lot of things. How efficient a person is in using their ki. (Ki can waste a lot of energy, or almost none.) How much ki a person can channel at once. How long a person can channel their ki.
At 0, a person is unable to use ki at all.
At 100, a person is basically able to use ki full-time. (Masking Ki tends not to be exhaustive so it doesn't really take that high of a ki mastery level to use masking ki full-time, but most who're skilled at masking ki tend to be pretty skilled at ki mastery anyway, but it's not a requirement. Basically, you can have Masking Ki be high and still use it constantly even with a low level of Ki Mastery, but most with high Masking Ki have high Ki Mastery.)
Full time, and in a diverse number of ways, and often mixing ki types. For instance, Whirlwind's technique of channeling one ki type to one hand and another to the other? That requires an innate level of Ki Mastery, because most people develop ki to use one type at a time.
The average ki user is 45.
Strength: The overall physical prowess of a person without taking into account ki. At 0, you get people who're extremely weak. Children, elderly, someone morbidly obese, basically, humans but humans who're not really able to do much of anything.
At 100, you get "basically a world athlete"; elite warrior level. Think like, the most incredibly fit military forces on the world. People who have practical strength. (For instance, a professional arm wrestler might have more arm strength in one arm than a professional soldier would...but that same soldier in all other areas is stronger; this level would be for the soldier, not for the arm wrestler.)
The average person is around 50. Fighters are rarely below 60, but not often above 90.
Speed: The overall quickness of a person without taking into account ki. This is in all aspects: their reaction time. How fast they process information. The speed at which they can strike. The speed at which they can move. At 0, you get children/elderly/morbidly obese, in that these people for different reasons are not in a condition where they can react fast, but they're still common enough humans.
At 100, you get people who aren't quite specialists, but are pretty up there: maybe not sprinting 10 meters in 6 seconds, maybe not running a sub-4-minute mile, but probably at least 10 meters in 8 seconds, and a solid 4:20 or so for a mile (assuming they have the endurance for that). When it comes to reactions, they can almost instantly see something and in less than a second have a response. These are things humans in peak condition who have bodies that are conditioned for certain responses are more than capable of doing. But it is exactly that. Peak human levels (and not in the comic book sense of the term), the best a soldier could strive to be, essentially.
The average person is around 50. Few exceed 80.
Intelligence: The basic smartness of a character. How perceptive they are, how well they learn, how good they are at thinking, the like. At 0, you get people who're Too Dumb To Live--and probably wouldn't be if not for outside factors having kept them that way. Basically, these people have less smarts than your average grunt; they just don't think about the logical consequences of things, have massively off ideas about reality, are real idiots and morons, yet still alive by some miraculous factor.
At 100, you get people who have a sharp mind: they are able to quickly pick up on things, they are fast learners, they process information rapidly, they have good knowledge, they are often able to make good judgement calls, they have a knack for the bigger picture and/or have a sharp eye for the finesse in things. They're just...smart people, really. Intelligence is a pretty self-explanatory stat.
The average person is around 50.
Dexterity: How finely a person can control things, essentially, without ki. Especially with their hands/wrists, but also elbows, shoulders, even toes/knees/ankles depending on the exact mechanisms a person can use in the art of war. Basically, this is how good a person is at maintaining what they want.
At 0, you get butterfingers: items slip out of hands, shaky hand syndrome is at its highest, stiff joints leads to poor flexibility, there's an inability for finer control of an object, it's difficult to impossible to make it function in certain ways which are theoretically possible, just...not with this person.
At 100, you get perfect control: every action is precise and on point. Double-jointed, even triple-jointed, in multiple joints. Bending, manipulating, basically someone can make anything do what they want, and even if it seems implausible for them to be able to get it to so consistently act that way, they control it accurately. (For instance, the famous Indiana Jones whip: high dexterity would allow it to always wrap around the desired target in the desired way, while also acting as a...well, whip when needed, and allowing to detach with perfect accuracy, and to swing with perfect accuracy, all stuff which might stretch plausibility but could theoretically be possible--and done perfectly 100% of the time. That's a 100 in dexterity.)
The average person might be 50, but the average fighter is at least a 70; it's pretty rare to see lower than 60. It is not uncommon to see many above 80. (Dexterity is something most fighters do incredibly well.)
Endurance: Both how much stamina a person has and how much punishment they can take, not to mention how easily they recover from an injury, all without ki. At 0, a person becomes winded easily and takes a long time to heal. There's not really much of a minimum in terms of how much punishment a person can take, realistically speaking, aside from perhaps slightly frail bones, easily torn muscles, the like.
At 100, a person can fight nonstop for days. They can go for a long period of time doing intense exercise. A long march. A battle after said march. Basically what your professional soldier does, only slightly more intense, the most severe of the severe in terms of what punishment they must take.
Furthermore, they can take a ton of punishment without going down--they're seemingly Made Of Iron. They can lose a lot of blood and not pass out (yet alone, die); they can receive blunt trauma after blunt trauma and not die from it; their bones are hard to break, the like. Now, without ki, even at 100, it's not hard to puncture skin. Flexing muscles can slow down a slice a little and bones can give resistance, but without ki, even with an endurance of 100, you're gonna die from a gunshot and gonna be sliced to ribbons. (Heck, even with ki bullets are gonna kill you.) But it does make ki much better at its job.
Additionally, at 100, a person heals pretty rapidly. Not supernaturally fast, mind you. But think, "can work out and then be fine a day later"; "wound that takes an average of two weeks to heal healing in a weak", the like. It's faster-than-normal healing, but still human healing.
The average person is at around 30. Most fighters tend to be at least 60, though few exceed 80.
Technique: Basically, overall technical skill. This is how much training a person has in their art, essentially. It can be refined by self-teaching, but most commonly is how much a person has been given formal training. There's a difference between being taught self-defense and just instinctively knowing how to defend yourself; they both work, but the better your technique, the better everything else is and it's usually best done from a professional.
At 0, someone has zero refinement: their attacks are crude and lack skill, but get the job done. There's nothing bad with having zero technique, there's just...nothing good. At 100, someone has perfected things, getting them as precise as is possible, with perfect training. They know every block, every defense, every move they can make, and how to execute those flawlessly 100% of the time because they have the training.
Everyone starts at 0. An average peasant who gains street-fighting experience is around 30. An average trained noble would be a 50. A trained fighter with experience is usually at least a 60. Most skilled fighters are around 70 (from lesser backgrounds) - 80, with it rare to see anyone exceeding 90.
Of course, these are all just approximations.
But anyway. That was me making an entry. I've got other stuff to do; it's taken me a few hours to write this.