Kesah, showing us all that fantasy barbarians are actually doing it right by running into armed combat without a shirt on.
The passive is unique and incredibly useful. However, I feel the particulars of it are too tricky for less number-minded players to see the comparisons of. Would it be better to build traditional AD items for their DPS-boosting passives, or tank items for the improved stat yield? Maybe half and half? In any case, it's not as obvious as, say, Jhin's attack speed conversion. I'm not sure how to make it more graspable, since it's honestly fine in its effect. It would just be harder for Average Joes to decide the build path for than say, Twitch or Fiddlesticks. Also, it's worth noting that Kesah does get bonus armor and mr from Items, it just doesn't factor into damage reduction because of the conversion, the same way Jhin technically still gets attack speed. That's obvious and there's no use saying so in the tooltip, it's just a point to keep in mind when it comes to things like Lethality calculations.
Vengeance is, I feel, a much more effective use of one of the concepts I feel you tried to get across in your Tuan concept. Rather than trying to redirect everything, Kesah effectively becomes a Braum wall that doesn't nullify the first projectile and that is both faster and smaller. This is good since Vengeance is both a defensive tool and an offensive one. Braum's Unbreakable and Yasuo's Wind Wall are purely defensive maneuvers, so they hardly need to move at all. Kesah's block range is restricted to his model, so it makes sense he be fast enough to still actually block things. One question though, when you say "damage prevented" you mean the 25/30/35/40/45% that was mitigated, right? Does true damage still get softened? If it doesn't, does taking true damage not add to that "damage prevented" then? Also, does melee and non-projectile damage that Kesah takes while Vengeance happens to be active add to it? Regardless of the answers to those particulars, the ability is still a very good one that allows Kesah to try his hand at behaving as a standard tank despite his passive, and to throw damage back into the enemy tank (who'll probably be the most convenient to hit in a fight) and help tankbust for the team to get into the backline. It gives Kesah a unique form of anti-engage, really.
I suppose, although it is not explicitly stated, that Burden of the Strong is an aura effect for things nearby, not a "mark everything nearby at the point of cast" sort of thing. That being so, it is useful if a little tricky to use correctly. It may be used to add to the protective screen of Vengeance and weather the storm of a fight's worst part, but then you're not reducing enemy resistances. Alternatively, it turns Kesah into a resistance-eating Pillar of Fuckyou that is running around in the enemy team, but then you'll probably be reduced to well-toned giblets thanks to your own reduced tankiness. In the end it is best used while the fight plays normally, when Kesah is trading blows with the enemy frontline in standard tank style, since it would help his team break the enemy tank sooner rather than later, like his Vengeance already does as well. Now that I think of it, it means Kesah has a unique identity of a tank-shredding tank balanced against the ability for him to be shredded first. He effectively speeds up teamfights, for better or for worse, by making the frontline of himself and the opposing tank die sooner than teamfights usually expect. Hum. That's actually a very neat concept.
The E is a sort of mini Sion ult with Highlander properties in its cc susceptibility. This is good as it is the main thing that lets Kesah truly be a functional tank by initiating fights or causing disruptions; something that most effective tanks have in some capacity. There's not much more to say about it, besides it being a decent getaway tool to help his place in toplane, where the laner is expected to be able to not die (be it through sheer bulk or nimble evasion). I do have questions, though. The limited turn radius is easy to grasp for Sion, who doesn't stop, but is a little harder to keep from feeling needlessly clunky if you can stop or be Rooted down while it's active. What happens if you turn into a wall? Do you just sluggishly turn while sprinting into it until you manage to right yourself (like a rushing Sun/Moon Tauros into the fences next to the Daycare where everyone with a lick of sense goes to run the Donuts of Egg-Hatching)? If so, an infinite duration means you can leave it on all the time out of combat, since the tripled movement speed is more than enough to open enough distance between yourself and pursuit to perform you less-than-stellar cornering maneuver. It also means that if you are Rooted down and CC chained to oblivion you'll still go from 0 to 731.6 as soon as they miss their mark, and if you turn on your Q while eating the chain you'll more than likely be alive enough to run aaaall the way back to base in roughly the time it would have taken to recall. In fact, in many cases, it would be safer to just E to base, because if you recall you give everyone time in your idle stance to maybe interrupt or pincer you, but with your E and occasional Q to failsafe you can almost always get back to your fountain. Perhaps some high max duration wouldn't hurt, something that would almost never be reached unless people were using it to run to the fountain and back to lane. Also some clarification on whether or not wall-turning works. As an aside, it effectively implements a clunkier version of DotA's turn speed mechanic, which is something that fundamentally sets DotA and LoL apart. DotA's turn speed means juking is harder, factors into why many of its abilities are point and click anyway, and is one of the many other degrees of complexity the game has over LoL. LoL doesn't have a turnspeed, which means many of its abilities are skillshots, since a lot of a skillshots' counterplay can come down to "don't get hit," which is a lot more doable if you can turn not only on a dime but on Roosevelt's temple. Just something to think about.
Keseh's ult is something like a Sivir ult, which allows him to initiate fights from far away in a similar manner to Kled. Not much to say on it, since much of what applies to Sivir and Kled would apply here. A useful ability and one that has great play-making potential. I am curious as to why Keseh's ult gives him and minions attack speed, but not his allied champions. That kind of makes it more optimal to use it to rush down a lane while splitpushing, and not really for starting a fight at all. Just something that gives the ability a lopsided feel is all.
All in all actually a very workable character, once the kinks on the E are addressed. One issue however is his similarity to Tryndamere in appearance. The shirtless barbarian with a sword thing is kind of already present. Perhaps a bulkier posture with a beard of a wholly different style, but also something that can make his model at a glance separate from Olaf's. I'd say drop the weapon entirely. It doesn't fit very well with the eschewing steel thing to be carrying a blade, and if his flesh is "hard as iron" he'd might as well fight bare-handed. The only other [humanoid] champion that does that is Lee Sin, and it wouldn't be too hard to shape Keseh the Beefcake in a fashion that sets him apart from Lee Sin (Vi punches, I know, but those aren't her bare hands). Also make his punches more in line with wild haymakers instead of actual martial arts moves.
Also: I'm going to read Baja next and if Baja isn't a girl I'm afraid you will have led me to my first beefy man on beefy man ship.
would bring him up to 731.6 MS with the point runner passive (1003.2 without the MS soft cap).
