Fanon Characters are characters that are not official characters from any fandom, but are characters created by fans, whether imagined as being a part of an established fandom or not. A character from the canon of another fandom is not a "fanon" character; if Uncle Grandpa, SpongeBob SquarePants, Harry Potter, or Robin Hood show up in the FusionFall universe, they are not fanon characters, even if introduced by fans, since they were created as original characters in the first place, and have their own fandoms. However, characters created by fans for other fandoms, who are then introduced into the FusionFall universe, are considered "fanon" characters, as are characters created by fans for FusionFall which did not appear in the actual game.
A Note on the Term "Fanon"
Strictly speaking, the term "fanon" implies that such characters are accepted by much or most of a fan-base as what one might call "semi-official" or "para-official"; though they are not actually part of a particular mythos's canon, in the feeling of many fans, they ought to be. An example of such a character is "Figwit," the name given by fans to a background character (played by actor and musician Bret Mackenzie) in Peter Jackson's film adaptations of J.R.R. Tolkien's The Lord of the Rings; there is no basis for such a character in Tolkien's text, nor in the film-scripts, but the character has become extremely popular among fans nonetheless. However, on this wiki, the term "fanon" is used to denote any fan-created content, whether that content is widely accepted or not.