Jonathan "Johnny" Test (voiced by James Arnold Taylor ) is a troublesome, narcissistic and widely disrespectful 11-year old boy who is the son of Hugh and Lila, brother of Susan and Mary and anti-hero of the show. Because he has Susan and Mary by his side, he can live any kid's dream, only to find that some dreams aren't worth living. He is usually seen with his partner-in-crime, Dukey, his genetically altered mutt. He also gives Johnny Coupons to his sisters for helping him in his daily problems. When redeemed, he does their tests for free.
He is very hyperactive, selfish and often messes with his sisters' inventions, causing trouble and mayhem, but just as often proves himself to be extremely clever such as by frequently tricking his genius sisters or saving the day from whatever danger happens to show up. Johnny can be considered very spoiled and stubborn, as he gets what he wants through deceit, blackmail, or manipulation, though he does love his sisters in a way only a brother can.