In Kingdom Hearts 2 when we are seeing Sora's memories there is a scene where we see Sora fight someone where he, Donald and Goofy fought Maleficenct's Black Dragon Form. the figure is wearing a black coat, much like those worn by Organization XIII.
this scene was not included in the original release of Kingdom Hearts but in the Japanese only Final Mix version which has now been localized in 1.5 HD Remix. in it this scene is apart of an optional super boss battle which the boss titled "Unknown".
during the scene Sora being voiced Unknown is not however Unknown comments that Sora is like it and is incomplete
a reference that when Sora was restored from a Heartless he is still missing the part of him which became his Nobody, Roxas.
Since Unknown must be a Nobody and resembles one of the members of Organization XIII, i am wondering if it's revealed anywhere who Unknown is (given that Unknown's fight is the only optional Superboss we see in Sora's Memories)