Rigby has all the fearlessness and shamelessness that come with complete immaturity. He's like an out-of-control teenager, with no boundaries whatsoever.
Mordecai is a young blue jay with a burgeoning sense of responsibilty, deceny, and his own mortality. But that doesn't mean he doesn't like to have a little fun every once in a while--stupid fun.
Pops might look human, but it actually a lollipop from Lolliland. He's been filthy rich and ridiculously sheltered his whole life, so he takes a childlike delight in virtually everything.
Skips can't walk or run; he can only get around by skipping. Both wise and strong, Skips can fix about anything Mordecai and Rigby break.
Because Pops is so out of touch with the outside world, and with reality, he relies on Benson to run the park. He's Mordecai and Rigby's boss and he's always on their case.