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.
Despite his flaws, Johnny is a very caring person, even if the situation he's in allows him to be selfish. A running gag for him is that he often feels bad about and hates seeing people cry, and tries to help them, knowing that will lead to pain and embarassment. He has felt bad for the numerous occaisions he's come out on top, such as when he defeated a desperate penguin army and help his arch nemesis return to his home planet despite the threat of him being double crossed and the Earth's destruction.
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.
His most recurring super powers include his ability to shapeshift, hurricane hands, and his most famous power: "lower posterior energy bursts" or "power poots," which are farts that turn into powerful flames. These powers have no status quo on usage. In one instance he managed to keep his "power poots" for a couple of weeks, while his other powers were neutralized, and in another, his mutant powers ran out due to exhaustion, despite this not happening the previously or thereafter.
One obvious aspect of Johnny's life is that the experiments he's the test subject for often lead him into trouble. Some of the outcomes have prooved to have the most astounding circumstances and be the defining moments of his character.
- He is the first person to discover real mole people, underwater civilizations and aliens, one of which he kept as a pet.
- He has fought two wars: one with penguins, the other with alien vegans, both of which he won.
- A switcheroo he imployed involving a hand-me-down backpack won his sisters a scholarship and popularity in a sorority. (On accident)
- He is the creator of the "Super Champion of the World Kart Race" and boat version, the latter of which he won and got recognized for on TV.
- He is an official secret agent and has stopped several world domination conspiracies.
- He has made enemies with several supervillains, all of which he's defeated.
- Saved the world dozens of times, most of which were his own fault.
- Defeated a few of his favorite video game protagonists and gained their glory.
- His sisters are the ultimate in genius prodigies of the time, and he has survived at least 100 experiments they've subjected him to, some of which were lethal.
- He has created two holidays: one was shut down for territorial infringement by The Easter Bunny, the other has succeeded and is carried out by one of his former arch-enemies.
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.