A.J. is a recurring character in "The Fairly Oddparents". He 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.
Personality
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. A.J. is much smarter than any other student, and this sometimes causes friction between him and his less intelligent friends, as he often flaunts his intelligence. A.J.'s family is very wealthy and they live in a big house, which is located near Chester's Trailer.
He appears to be slightly chubbier than both Timmy and Chester, suggesting he eats more (possibly due to his higher living standards) or he has inheirted some his father's gurthy genetics.
Weight Gain
A.J. has only gained weight once in the series canon.
In the episode "Just Desserts" when Timmy wished that every meal was dessert, all the kids, including A.J., became enourmously overweight.