Bunnicula is not a main character in the books, he doesn't talk or babble or anything at all, he is a pet rabbit. May or may not be a vampire but signs point heavily to yes, has very mysterious origins.
Chester and Harold argue because Chester wants to kill Bunnicula and Harold doesn't.
Both are smart in the books and can read and write, but Harold is laid back and Chester is neurotic. Harold is also lazy and Chester is obsessed with the supernatural and a would be vampire slayer type.
They have a little friend named Howie who might be part werewolf.