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

Nasir is a laguz (from the dragon's tribe) who is entrusted as a guide for Ike's group throughout their travel to Empire of Begnoin. He appears to be a merchant but he's secretly working, as a spy, for Dain. He's also the one who stoles the Fire Emblem from Mist and he also protects Ena when the group reaches the enemy's capital, revealing his true identity and his real "side".
Anyway Nasir is a good person. He really despise himself for his actions, which are only lead by the love (and sake) he feels for Ena, his grandaughter. According to the development of the events (if you win or not the fight with the Black Knight), he joins your group (if you win) or dies in the attempt of let you escape from the battle.
Ike forgives him in any case, since heunderstands his reasons and he has always seen him as a friend, despite his actions.
Nasir is a secondary character, but probably, on the psycological side, he's the most well developed one.