Francis is extremely dim and unfriendly. He tortures Timmy and his friends for his enjoyment, ranging from giving them wedgies to outright beating them up with his fists.
Shown to be usually very stupid with a tendency toward unprovoked aggression, he represents a stereotypical bully. Despite the interactions, Francis is shown to have a caring side sometimes, although it's mostly when he is alone.
Francis has only gained weight once in the series.
In the episode "Just Desserts" Timmy wished that every meal was dessert, causing Francis and the other kids to become very fat, except for Mark Chang. Because Mark wasn't fat like everyone else, Francis made fun of him for being thin and even attempted to shove him into a locker (because he would fit) but couldn't because of his lack of energy.
In the original script for the episode (which can be seen here:http://www.scribd.com/doc/15589147/223-Just-Desserts), Francis and Tad mocked Mark even more before Francis managed to shove him into a locker. And later on, Francis tired to put Melvin in a locker before Timmy reversed the wish.