Alonzo is the second in command of the Jellicle Tribe. He will take over Munkustrap's duties when Old D. leaves for the Heavyside Layer and Munkustrap becomes the leader. This is surprising because Alonzo wasn't born a member of the Jellicles, but a stray that later joined the Tribe. His parents are Skimbleshanks and Vau, a member of the Mountain Tribe. You can read his story up until he joins the Jellicles in an upcoming fanfic.
He has only one sibling that he knows of, Pouncival, but he's always on the lookout for other members of his family. However, right now Alonzo is content to stay at home in the Junkyard with Jezzie and his friends.