A.J. is one of Timmy's two best friends along with Chester. A.J. is smart and rich, but is not very good at social skills and as a result he is considered an unpopular kid like his two friends.
A.J. is considered a super genius by his friends. He is the only student that seems to ever pass tests in Mr. Crocker's class. He usually wins the school science fair as well, even when up against Timmy and his fairies, because fairies cannot help win competitions. A.J.'s family is very wealthy and they live in a big house, which is located near Chester's Trailer.
Although A.J. doesn't have a girlfriend, he did once have a friendly exchange with Trixie Tang when he offered her a dead frog and she found it "neat", although Chester dragged him off before he could make anything of it. Another time when Trixie was getting angry that Timmy was ignoring her ignorance, A.J. said that Trixie could ignore him all she wanted. He usually scoffs at Timmy's own attempts to woo Trixie Tang, but possibly has a crush on her himself, in the episode Emotion Commotion he wanted to be "the best-friend of Timmy's she gets with to get back at him", but she goes with Chester instead.
Francis bullies A.J. much like the other smaller kids, but A.J. sometimes uses his inventions to defend himself. An example of this being the AJ-5000, an artificial "big brother" that he created to protect himself from bullies.
He is bald, and wears a light blue sweater vest, which Timmy describes as a "girl magnet", over a white shirt. He also has gray pants with black shoes. His pupils are colored blue. A.J. is around the same height as Chester and Timmy. In "It's A Wishful Life", it is shown that if Timmy didn't exist, then A.J. would have a full head of hair.