Ichigo, duh.
so this is what I got from the past few chapters.
Shunsui's brother marries Nanao's mom
His brother dies because of the "Ise Curse"
Nanao's mom begs Shunsui to take her zanpakutou in order to spare Nanao from the curse, since she thinks it has something to do with the sword
Nanao's mom gets executed for losing a sacred treasure of soul society
Shunsui is burdened by the fact that people he care about entrust him with their precious things, and wants to help Nanao to lighten his burden (I assume the burden she put on him of the possibility of her death).
so yeah, that's what I got. I can't discern when exactly he became a captain of the gotei 13, but I think it was after the mom's execution, since Nanao makes note of his kimino and how it looked like her mom's.