Chōjirō
Sasakibe

Chōjirō Sasakibe was Genryūsai Yamamoto's lieutenant for over a thousand years and the first student he ever took. He is a quiet presence in the background of nearly every captain's meeting, and for most of the series the audience knows almost nothing about him beyond his loyalty and his moustache.

He mastered Bankai before Jūshirō Ukitake or Shunsui Kyōraku did, and he never used it in front of anyone in the entire history of the Gotei 13. Kōkō Gonryō Rikyū calls down lightning across the whole Seireitei. He kept it hidden because Yamamoto had told him his Bankai was like a child's, and Chōjirō agreed, and decided he would spend his life improving it rather than displaying it. He turned down substitute captaincies for the same reason.

The Wandenreich kill him in their first invasion, and the reason is exactly that secret: Sternritter stealing Bankai cannot take what has never been seen. He releases it in front of them and dies, and Yamamoto's reaction on being told is the angriest the old man is in the entire series. People had insulted Chōjirō for perceived weakness for centuries. He was the second strongest man in the room and had decided it was not his place to say so.