It must be a canon AU (so for instance Cloud from FF7 and Cloud from Kingdom Hearts, but not Cloud from a fanfic). The exception to this is you may app a canon character having been in the game multiverse. That character's slot will still be taken. So for instance you could apply KH!Cloud as having stumbled across a multiverse gate and bummed around selling his skills to the Fay'lia and other factions as a mercenary. KH!Cloud would then be listed as taken if your app was accepted.