He is overweight and middle-aged.
He has an unrequited interest in Velma Dinkley, a motive to set the gang up as criminals.
What's New, Scooby-Doo?
He delivered Valentines to them, and also some extra messages from a "Secret Admirer." Because that mail was actually from the mastermind of the evil clones he was a suspect, but he turned out to be innocent.