Lemme guess. He's a soldier with a dark past, who was part of a secret division. Or he's an assassin.

if he's an assassin, and he's around the same age as Karite, we can have some nice fights...
The ironic thing is, NOT AT ALL!

Basic concept brewing in my head.

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He was an extremely talented heart surgeon whom had never lost a patient in 25 years of his career. However, one day, a mishap caused him to lose his cool, and subsequently his patient. He drank himself into a stupor, and ended up losing his family, friends, and everyone near to him, dying alone. Then he decided to haunt the hospital he used to work in, which is why he attracted Shinigami attention. He went to Soul Society, but for some reason, upon getting there, he lost his memories of his previous life, and reincarnated as a child. I'm not sure how to add a turning point to make him into a blood knight from there, but I'm getting there.