My complete theory based on the Birth By Sleep secret video

I think that Master Xehanort is Ansem's apprtentence.

The video is a memory of Xehanort.

Xehanort was in the Keyblade War.

The desert area is where the Keyblade War took place.

Chasers are keyblade wielders who are sent to hunt down other keyblade wielders who have gone rouge.

Terra, Ven, and Aqua are chasers who were apprentices of Xehanort before the apprentice that dissapeared along with Xehanort.

Mickey had attacked Xehanort before he could get into Kingdom Hearts, resulting in Xehanort's memory loss, Xehanort getting younger, and Mickey receiving his golden keyblade.

I also had thought that Kingdom Hearts should have a king who is the light, the voice talking to Sora in the first game, forged the keyblades, and all of the keyblade wielders are like his knights.

The Keyblade War resulted from some of the keyblade wielders being corrupt by darkness, possibly becoming heartless like Xehanort's heartless from the first game.

All of the worlds were once one world that had split because of the fighting over the light mentioned in the first game.

I think that is about it. Please tell me what you think.

 

Well, I think that they were his apprentices at one time or another.

 

It's a possibility