Isshin
Kurosaki

Isshin Kurosaki runs a small clinic in Karakura Town, ambushes his teenage son at breakfast with flying kicks, and holds loud conversations with a poster of his dead wife. He was also, under the name Isshin Shiba, the captain of the 10th Division and the head of a branch of one of the Five Great Noble Houses.

He lost the rank chasing an anomaly into the Human World: a Hollow called White, built by Aizen out of Shinigami souls, hunting Quincy. It attacked Masaki, a Quincy, and Isshin could not save her without giving up nearly everything he was. Kisuke Urahara offered him a way to do it, and Isshin took it knowing it meant surrendering his powers and his position permanently. He married her, and they raised three children in a town he had no authority in.

He does not tell Ichigo any of this until the boy has already fought his way through Soul Society, Hueco Mundo and Aizen. When he finally does, in one sitting, it reframes the entire series: Ichigo is not a Shinigami who picked up a Hollow, he is the child of a Shinigami captain and a Quincy, and every strange thing about his power has an explanation his father was sitting on. Isshin regrets the timing. He does not regret the trade.