He is frequently called the "kid with the flaming hair" due to having scarlet red highlights in his yellow hair, which sticks up like Calvin's (of Calvin and Hobbes). As well as this, he is most often seen wearing green cargo pants and a black shirt with a trefoil symbol on it, which he wears under a navy blue dress shirt, and a watch that he rarely uses. He sometimes wears orange, yellow, and dark pink swimming trunks and his disco outfit was a black shirt, dark gray pants, a black cap, blake glasses, a yellow collar, and black shoes.
He even owns what appears to be a red and black B.C. Rich Warlock guitar that is rarely ever used or seen, Although it can be seen in the intro played and smashed by Dukey and played by Johnny, Dukey, and Bling-Bling Boy in very few episodes. 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 very hyperactive, 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.
Johnny has an alter ego that he calls "Johnny X", who is a superhero that has various superpowers. A running gag for this form is that his hurricane hands never work, due to either the target being immune to it or to other unforeseen factors.
Johnny hates school and doesn't work hard at all; if anything, he goes to great lengths to avoid doing work, often using his sisters' inventions to do so and often putting himself and/or others in danger as a result. He is addicted to video games, and will do anything to get them. His catchphrases are "We did not see that coming" and "Hokey smokes." which are usually used during an unexpected event. There have been minor alterations to that phrase and in some cases, others have said it, including Dukey. Johnny is of English, Spanish, Austrian, Canadian, Scottish, French, Swedish and Japanese heritage as seen in Who's Johnny?. Johnny has a crush on his sister Mary and Dark Vegan's daughter Jillian.
- He is based off of Dee Dee from Dexter's Laboratory.