I suspect that the biter is a male puffer by the way he acts. He is smaller then my other puffers and has exhibited mating behavior (chasing and nibbling and displaying to the female still in the tank) However I think he looks like a female to me - I don't notice the eye wrinkles, no belly stripe and his coloring seems the same to me as the females. If it wasn't for his behavior I would totally think he was a female. I've added a picture of him to this post so you guys can tell me what you think. If it helps I've had these puffers for 8 months.
What I'm trying to figure out is if this sudden aggression might be mating behavior or simply too many puffers in the tank (3 in a 20 gal)? Should I attempt to reintroduce the bitten puffer at some point? (I'm leaning toward keeping her separate) And lastly, if you guys think my puffer is really a female, is it common for a female to take on the male role in mating behavior? The first picture is the puffer in question, who acts like a male but I don't think looks like one, the second picture is of my female who is still in the tank with him.
It's actually 2 pieces of driftwood (and a piece of dragon stone that was holding one down until it became waterlogged but I never took it out 
) I purposely picked ones that would have lots of nooks and crannies to break up the puffers view of each other.