I heard this online buut it makes sense. Xehanart (the student of Ansem) might have come into exsistance from Terra and Master Xehanart mergeing some how. The hair(Terra) the hair color(Master Xehanart) The clothing(Master Xehanart) and if Xehanart becomes Xemnas along the way he would have a reason to call Aqua old friend.