Ashido
Kanō

Ashido Kanō is a Shinigami who has been alone in the Forest of Menos for several hundred years. His unit chased a group of Hollows through a Garganta into Hueco Mundo and never came back out. They kept fighting until there was nobody left but him, and he stayed.

What he has been doing since is killing Hollows. He wears a fur coat with five masks fixed to the back, taken from the ones he has put down, and a gazelle skull over his face, and he moves through the forest like something that belongs to it. When Rukia Kuchiki falls into the Menos Forest and he pulls her out of a fight she cannot win, he tells her flatly that he underestimated her.

He appears only in the anime, in a short arc that does not connect to anything else, and the series never resolves what happens to him. He refuses to leave with Rukia because the forest is a nest and somebody has to keep the numbers down. The character works because that reasoning is not heroic posturing: it is the only structure a man has left after several centuries of outliving everyone he arrived with.