Grodd is the one that surprised me too. I could see perhaps, since the Joker IS really charismatic, the low-powered or gimmicked-weapon using villains siding with him, but a high level psionic like Grodd? Nahhhhh.

But then again, this is IIRC the story where they decide to torture Killer Frost by dangling her feet into a bonfire as if to melt her... which is absolutely the stupidest thing you could do to Killer Frost, since her cold powers actually work in line with science; she doesn't generate cold (Which on one can do) she absorbs all thermal energy from around her.