A fanlisting is simply an online list of fans of a subject (in this case for Celine Jules) that is created by an individual and open for fans from around the world to join.
There are no costs, and the only requirements to join a fanlisting are your name and country. Fanlistings do not have to be large sites (although some are), they are just a place where you can sign up with other fans. TheFanlistings.org is the original (but not official) web directory for fanlistings, dedicated to uniting the fans.
Description from TFL.

Age: 23
Birthday: September 18
Height: 5'6"
Weight: 110 lbs.
Weapons: Rods
Fav.Food: Baby Rabbit Risotto
Fav.Instrument: Violin
Seiyu: Kana Koinuma
Bg: Celine is an heraldic mage from the village of Mars, located on Espell. She is a sort of treasure hunter, still she isn't so much interested in making money, but in buying anyhing that suits her expensive tastes. She might appear as a sarcastic and crafty person, but instead she has deep feelings and a kind heart. She is a very important character especially at the beginning of the game, but she's also worth to be part of your ultimate party (I think she's the best mage even if she hasn't a powerful spell like "Extinction").