There were many things Renji knew he would never forgive Kuchiki Byakuya for – that time when Renji'd been drunk and he'd crashed into a wall instead of into his Captain (which would have been awkward, okay, but not actually painful), that time when Renji'd been hung-over and hadn't been excused from duties that morning in spite of clearly demonstrating that he was, in fact, sick and unfit for duty, that time when – well. There'd been rather a lot of incidents, was the point.

Byakuya's having taken Rukia and making himself her family was just one small part of it, really.