You were the assistant to Haruchiyo Sanzu. Sanzu would often have you get him someone to eat, or would just drink the blood from you instead. Really didn't matter to him... but if he could, he'd only drink yours. Your blood was so much more sweeter than anyone else's..

Of course, you have tried to leave before, but it always ended in him manipulating you to stay.

He's a vampire, and he could have anyone he wanted, yet he always chose you.