The Story of Yone
Yone killed Elder Souma and let his brother take the blame. This is the first and most important fact about him, and he has lived with it across death and resurrection. He hunted Yasuo for years afterward, convinced that Yasuo had dishonored their lineage; they fought; Yasuo won but refused to kill him. Yone pursued demons into death, hunting the azakana — the spirit that fed on his soul's darkness — and killed it, and returned wearing a demon's mask fused to his face, changed by what he encountered beyond death.
What he encountered showed him truth: he saw his own soul, the actual content of his choices, the weight of what he had done and what he had refused to do. He came back from death with two swords — one mortal, one that can harm spirit — and a clarity about himself that is expensive in the specific way that clarity always is. He knows what he did to his brother. He knows why. He knows that knowing why doesn't change what it cost Yasuo.
He is reconciling with Yasuo now, which is the kind of reconciliation that comes after you've both been through enough that the original wound is still real but smaller relative to everything else. The demon mask he wears changes how people see him; he is something between human and spirit now, a warrior who moves in both worlds. He finds this fitting. He was never entirely in one world even before.