First Appearance

Xemnas first makes his true appearance known in Radiant Garden (Still known as Hollow Bastion at this time).
As he takes off his hood, and turns to face Sora & co, King Mickey immediately realises whose nobody this is.
We are then taken to a short flashback, featuring King Mickey talking to Ansem the Wise. After pointing out how dangerous his studies
are, Ansem agrees to halt the experiments. Xehanort then comes through, and seems to disagree - He believes the experiments should continue.
After giving up a short arguement, he bows and leaves the room, bringing us back to Xemnas. Xemnas begins to smile with evil intent at Mickey, and Mickey brings out his Keyblade.
As he rushes to go and meet Xemnas personally, Sora & co are left with some heartless to fight.
-------------------

When the heroes reach Xemnas, King Mickey is already there, waiting behind Xemnas. When Sora calls for Mickey, he calls to Xemnas by his old name, Xehanort.
Xemnas replies by asking himself how long it's been since he abandoned that name.
Sora then throws two questions at him - Where is Kairi, and where is Riku. Rather than call him by a name, he just calls him 'Nobody'.
Xemnas replies calmly that he knows nothing of any 'Kairi', but as for Riku..... His King should know.
Xemnas begins to leave, but as if King Mickey didn't want to face Sora, he chases after Sora, leaving the Keyblade Wielder with no clues as before.