"And the backstory for Hammer Girl and Baseball Bat Boy was like this- so every night, their father would sit them down at a small table and pull out this coin that he would spin on the table. And for whichever side the coin landed on that particular evening, that was the child he would then beat that night. So that coin represented pain. One fateful night, it was Hammer Girls turn to get beat but Baseball Bat Boy flipped it over to his side so that he could take the punishment. Their father went kind of nuts and beat them both so severely that he damages her eye in the process and they, in turn, find strength together and beat their father to death after that. So thats just their origin story and all the characters in the movie had these kinds of stories too. Maybe well do an origins comic book for these guys though; I think that could be fun."
source:
http://dailydead.com/exclusive-interview-raid-2s-gareth-evans/