From left to right:
Solomon: Rejected by fellow skeletons because of his crazy hight, he started hanging out with the zombie-boy Xavier. He taught him how to survive the sunlight and humans.
Andrew: Both his parents were murdered for their pearls, so he built himself a shelter out of random blockes that interested him. A creeper allmost mistook him for a human because he's still just a kid, but when they figured it out, they became good friends.
Craig: Spawned in a dungeon deep underground. A human tunneled into it and promptly died. He followed the tunnel to the surface and decided he wanted to live up there. Hasn't blown up yet because he's very intelligent.
Xavier: Spawned one night in the middle of a crazy zombie party. When the sun rose, he barely made it into water in time. He met Solomon by chance under a shallow overhang. Andrew met him while teleporting away from water.
Might make a comic for these guys.