So who is it? That person (or pet) that thinks you're the greatest? That person (or pet) that thinks if you change the channel on the TV it's "OMG! THE GREATEST THING EVER!"

For example:

Stephen King had pretty much given up and threw his manuscript of Carrie in the trash. But Tabitha dug it out, thinking it was great, then started mailing it in to publishers. It launched one of the most recognizable writing careers in current history. So one could say that Tabitha is/was his biggest fan.