1, 2, and 3 for the crucial muse questions. :3
1 - What would completely break your character?
Well - first and foremost, Black Wind is a warrior, tempered by the harsh struggles of his childhood as well as his calling as a summoner and a de-facto soul mage - he knows very well what mortality entails, even if fusing to Magun rendered him mostly unkillable, a so-called "Unlimited".
He handles the spirits of the departed everyday in the form of the Soil he uses as summoning ingredients, and as such has an intimate relationship with death and didn't hesitate to give his life for Windaria, even back when he thought the sacrifice of his physical body would kill him like any other human. No, it is not death which scares him, not his own or those of his comrades, as much as he's still haunted by the meaningless loss of his people. So, what is it that truly terrifies Kaze? The thing that would ultimately be enough to break his iron will?
Have you ever seen just what Chaos does? Can you trust the souls absorbed into the abyss to ever have a chance to rest in peace? Chaos is a thing which enslaves you beyond death, strips you of your mind and purpose. Not even summoners and their called beasts are safe.
None are, truly. Even the Magun itself, and by extension, Kaze and his Bombardment Beast form were possessed by the demon at one point, made to kill innocents like some deranged puppets. See, Chaos may not have been able to destroy them, but it broke their mind and severed their connection which led to Black Wind losing his memories, sanity, and the ability to fire the Demon Weapon at will. He was a shadow of his former self, a confused mess trying to murder his only friend or to lose his own life trying. Eventually, he recovered, but the experience torments him even after all these years.
Even as an impoverished child, ever on the verge of starvation, he had always held complete control over his mind and soul. The two things he took for granted, and they were taken, if only temporarily; It shook him to his very core. He isn't sure whether he could bear it were it to happen for a second time, especially if his failure doomed more souls to the same fate, a fate nobody could wrench them away from. He no longer cries over his sister's death, no, he cries over what comes after.
He never believed in happy endings, but Chaos shattered the few ideals he did harbor, and for that grave wrong it will always be relentlessly pursued and punished by the gunslinger, to the point of his very name taking the back stage as he adopted the alias of "The Hunter" as his new and final mantle, with the words "Black Wind" reserved as a secret revealed only to those worthy of his respect.
2 - What was the best thing in your character's life?
Away from all the doom and gloom now! ^^ The best thing that happened to Black Wind...
Honestly, probably finding his place in society through being adopted by his clan. That is also where his facial markings come from. He had been trying to get noticed by the Wind Warrior caste in hopes of training as a summoner, and his persistence and natural aptitude for combat has seen his dream fulfilled. Who knows, if he hadn't been taken in that day, he well may have died in the scorching heat without money, water or shelter.
Never having known his parents, the Warriors were his first family, and he loved them dearly. Without them, he wouldn't have had the opportunity to pursue his ambitions and end up as a general, a war hero, and eventually, the next wielder of Magun. If not for that amazing chance he was granted, Windaria may never have seen its own Unlimited. Makes you wonder just how many prodigies were lost to the slums of his homeworld, denied a chance at life by the circumstances of their birth, much like what happens all too commonly here, on Earth, as well...
3 - What was the worst thing in your character's life?
And we are back to point one, aren't we? In this case, the points overlap. He may have been a street urchin, some of the abuse he suffered scarring him for life, shaping his personality into what he is today - a quiet cynic with little regard for ideals, but none of what he had to endure could ever compare to the horrors Chaos inflicted upon him and his people. Nothing ever will. That is the reason he fears nothing and no one, save for the sea of teeth and eyes which ravaged his world, his soul, and the futures of too many others to count.
All things considered, Chaos is his sole quarry, and he will stop at nothing to destroy it and spare others from witnessing the terror it brings. That is the essence of who he is, a lonesome warrior, the last of his kind, and a perfect hunter’s weapon fused into one and set on a neverending quest. That - is the nature of Black Wind.
[Thank you for asking! Always happy to analyse this character further. ^^]