Don and Rob are a goofy pair of Mutants who provide comic relief and give a human face to the gang. After the stalemate at the dump, they go back and forth over who to follow - Batman, or the Mutant Leader. "Don't shiv" is Mutant-speak for "doesn't back down," or "isn't messing around."
While the general public debates Batman's criminality, the Mutants are impressed by his mystery and power. At the rematch, they crowd around like fans at a sporting event. "Balls" is the name of one of the senior Mutants.