A fanlisting is simply an online list of fans of a subject (in this case for asch) 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.

asch

basic info

One of the Rokujinshou, the commanders of the Lorelei Church's "Oracle Knights". As leader of the special forces, he accepts a secret mission to observe and interact behind the scenes with Luke and company's journey. He fears noone, and is arrogant and self-righteous, but his skill with a sword is first-class.