A persistent disturbing preoccupation with a celebrity, i.e. actors, actresses, muscians, sports stars etc.
She tried to collect all of the bands sweat towels. She is an obsessive fan.
Obsessive-Fans are the scary kind of fan. They know EVERYTHING about their fandom. They have it memorized. Obsessive-Fans blow a fuze if they miss a single movie, blog, T-shirt, etc, of their fandom. They take it RIDICULOUSLY serious.
Obessive-Fans are avoided by Proper-Fans and Casual-Fans, though Psuedo-Fans latch onto them in an attempt to look like Proper-Fans.
Note: It is possible to be an Obsessive-Psuedo-Fan, becoming Obsessive with every new trend. It is also possible to recover from being an Obsessive-Fan, though it is usually very difficult.
Jill is an Obsessive-Fan of Twilight. She knows every speck of trivia, and if someone so much as gives her a weird look when she's going on about Twilight, she'll have his head. Literally. Jill's room is full of Twilight posters. She has mutltiple copies of each book. EVERYTHING in her life revolves around Twilight.