The fan character is similar to an original character, but unlike the original character, s/he might be written by more than one person. In these cases, the fan character is no longer original to one author but now used by the canon's fandom as a whole. Herobrine, from Minecraft , is an example of a fan character.
Sometimes it might be difficult to do a mission that involves a fan character as compared to an original one.
Caution: In some circles, the term "fan character" may simply refer to a fan-created character, in which case it means exactly the same thing as "original character." In that case, the character's creator might get annoyed if you try to use said character without permission.