Library Keeper and Guardian
Master Yen Sid protects the library from the outsiders with a hidden barrier that conceals the library from non-Keybladers (e.g. - nobodies, heartless, etc.) While Yen Sid is away, I'm here (Keyblade Master Adalene "Adale" Lynahart) to guard its secrets against those who want to know too much about the Keybladers strengths and weaknesses (e.g. - Xehanort).
Master Yen Sid used to be a Keyblader. This is an unknown fact, but I think King Mickey inherited the Keyblade from Yen Sid. That may or may not be true 'cause Master Eraqus, Master Xehanort, Master Yen Sid, Terra, Mistress Aqua and Ventus "Ven", all have Keyblades unassumed of having inherited it from another Keyblade wielder; unlike Sora, Riku and Kairi.