Bleach conjectures (spoilers!)
So, I keep thinking about the Kaien-hollow. I think it is probably either a dual personality arrancar-ish thing, with Kaien and the hollow fighting for dominance, or it is an Espada that Gin sent particularly for Rukia cuz he LOVES to torture her. He has some sick, twisted, obsession on Rukia, ever since Gin became a captain and began those creepy talks with Byakuya.