Protégé by black.k.kat.
Yamamoto Genryūsai Shigekuni is, at heart, an old man who likes his tea prepared a certain way and has a soft spot for talented youngsters. His latest fledgling seems to be one Kurosaki Ichigo, substitute soul reaper. (Alternatively, a story in which Yamamoto is a crafty old man, Ichigo grows up to be utterly terrifying, and Aizen doesn't stand a chance.)
https://www.fanfiction.net/s/9896876/