Chambers of Repose and Waking

The Chamber of Repose (眠りの部屋) and the Chamber of Waking (目覚めの部屋) are mysterious rooms that seem to hold some great importance to Xemnas. Whatever that purpose is remains unknown, but future titles should explain the origins of the chambers.

Kingdom Hearts Birth by Sleep
In a recent trailer for Kingdom Hearts Birth by Sleep, Ventus is seen sitting unconsciously in a chamber that is extremely similar to the Chamber of Repose.

Kingdom Hearts 358/2 Days
The Chamber of Waking is supposed to contain Xemnas's other "friend", who is suspected to be Ventus. This room is said to be found in Castle Oblivion. Whether or not Xemnas ever found this room is unknown, although based on conversations between Saïx and Axel in Kingdom Hearts 358/2 Days, it is unlikely that Xemnas has, since Axel was ordered to search for the room after he had disposed of the Organization's traitors, but failed to find such a room.

Kingdom Hearts II Final Mix
After the apprentices banished Ansem and became Nobodies, they found the Chamber of Repose within Radiant Garden's castle. Xemnas would frequently travel to the room, as if something within it reminded him of his forgotten past. In the scene in Kingdom Hearts II Final Mix, Xemnas entered Ansem's computer room, inserted a data disk into the machine, and typed in the password "Another", after which he typed in six more passwords: the original names of the six apprentices.

This caused a passage to appear below the Heartless Manufactory, which had a long stairway spiraling down to a long hall. Inside this hall were doors electronically locked and bound with chains. At the end of the hall was a room containing a throne for Xemnas to sit on, as well as the armor and Keyblade of a warrior named Aqua. Xemnas would speak to the armor, addressing it as a "friend".

Apparently, Xigbar often eavesdropped on the conversations, and told Zexion that he heard another voice once. He also told Zexion he suspected that there was a link between the Chamber of Repose and Castle Oblivion, where, allegedly, the Chamber of Waking is.