Bayou Da Vinci by David Pierson

But there is a problem with the man because he is actually three people in one!
In addition to his "Bayou Da Vinci" persona, there's also his madcap alter ego, "Mr. Z," who often takes over in the middle of Wally's classes. Mr. Z doesn't talk to animals, inanimate objects and invisible people; he argues with them. However, he makes too much sense to be called crazy but is far too crazy to be considered sane.
And then there's "I." A dark soul, "I" is the identical twin brother of Wally (who doesn't have a twin brother). "I" is the mirror image of Wally - except for the scar on his lower lip. This scar, which, instead of being on the left side - like Wally's - is on the right side. Something else about the scar: It is more pronounced on "I," as if it hasn't healed as well as Wally's scar.