Blood Moon!Yasuo has a relatively humanoid appearance; the only distinct thing(s) about him would be elven/pointed ears and sharpened fangs. i would suppose he has a “true” form, but i have yet to decide on what that’d look like.
Yasuo’s mask is not like the mask in game; his mask bears the appearance of a kitsune ( just like THIS. )
his point of origin is not fully determined. the tale goes that he lends his aid to humans he deems worth his time, winning their victories and shedding blood in their honor.
estimated 600+ years old, appears to be in his mid-20s.
this is based on the 2014 Lunar Reveal and goes hand-in-hand with the Tryndamere and Diana skins. the idea is that the three of them are competing for control of the land, and each have their own motives. for Riven, this is to keep her people safe from the likes of the other two.
earned her nickname ‘Dragonblade’ after, of course, slaying a dragon, a task many considered impossible. it was this feat that put her in charge after the previous general passed from combat.
she travels across the country in hopes of recruiting the leaders to join under her for protection. she treats those who do not in a very Kuvira-like fashion: she will go to war, slay the opposing army, then forcibly take them under her wing. really, she gives you no choice in the matter, except for whether or not you’d like to do it peacefully :^).
about them both:
after deeming her worthy, Yasuo lends Riven his aide in gaining control of the land. there are times where she need not even raise her blade - by the time she arrives to the scene, he’s done all the dirty work, and the leader is ( usually ) ready to re-negotiate.
their ‘relationship’ starts as a predominantly sexual thing, one where both satisfy each other. it later develops for time; for Yasuo his intrigue in her fiery personality and her ambition grows. for Riven, his steadfastness, dependability, and loyalty to her strengthens her fondness for him.
demons enhance inherit ‘sin’. for Riven, her greed becomes more apparent with time, often ordering for wanton destruction when there’s no real need for it. this in turn feeds Yasuo’s own desires for killing, and pleasing her.
i also have a sort of ending scenario with @hvsagi where demons can extend their lifespan onto mortals they grow fond to, but it is a one-time process and naturally, few are willing to do it. in this case, Yasuo does it without really realizing it, but decades pass and Riven never ages, never weakens. it is said that there is an eventual collapse from all the warfare, and the two seem to fade out of existence. those who desire their power in times of war, however, may be able to call upon them once again.