Well, let's see: Zer0 is much stronger than his body type would suggest (he's very tall and thin, yet probably as strong as Salvador, a steroid abuser). He's only got 4 fingers per hand (3 fingers + thumb), with no evidence of amputation of any sort. Even Handsome Jack, with all his security cameras everywhere, cutting-edge tech, and all that, can't find much more on Zer0 history than a shrug and an "I dunno."