Prince of Underwhere by Bruce Hale

It's tough to be ZeKe.

He's got his hands full: There is his prissy, know-it-all twin sister; his mean cousin Caitlyn, who's house-sitting for his missing parents; and a bully making life tough at school (as though it wasn't hard enough already). And now, thanks to a stinky, scruffy, good-for-nothing talking cat, he's also got to cope with zombies, midget freedom fighters, devious spies, superstar rappers, and a whole weird world beneath our own where people wear their underwear on the outside of their clothes.