Kaien
Shiba

Kaien Shiba was the lieutenant of the 13th Division under Jūshirō Ukitake, and the man who taught Rukia Kuchiki what a Shinigami is supposed to be. He was loud, direct, and completely uninterested in the deference her adoption into the Kuchiki family had trained into her. He told her that a heart is what forms when people are together, and that she should stop apologising for existing in a room.

He was married to Miyako Shiba, the division's 3rd Seat. When a Hollow called Metastacia killed her and her body was returned, Kaien refused to wait for an assault squad and went after it himself with Ukitake and Rukia following. Metastacia's ability was to fuse with a Shinigami's soul, and it took him.

What the series does with that is the reason it matters. Ukitake attacks the thing wearing Kaien and then collapses mid-fight with his illness, and it turns on Rukia, and Rukia runs Kaien through with her own blade because there is nobody else left standing. He thanks her with his last breath for letting his heart stay in the Soul Society rather than in the Hollow. Byakuya Kuchiki notes later that Ichigo Kurosaki looks a great deal like him, which is not an accident of casting.