Well Steve Purcell isn't the main character of his comics.
Also I'm not sure Jon was ever supposed to be based on Jim Davis. According to wikipedia (http://en.wikipedia.org/wiki/Jon_Arbuckle
Jon is based on an employee of Paws Inc. But I'm confused because I thought Paws Inc. was created so that Jim Davis could just pay other people to draw Garfield for him for the rest of his life.