Ventus, Terra and Aqua are from the FUTURE!Edit
Ventus, Terra and Aqua and all of the Birth By Sleep characters are from the future. Terra is actually the desendant of Riku, Aqua is the descendant of Kairi and Ventus is the descendant of Sora or Roxas.
Well, some time in the far future, Master Xehanort tries to start another Keyblade war that was from possibly hundreds of years ago. Master Xehanort then finds out that Ventus, Terra and Aqua will one day defeat him but tries to kill them while undergoing training from their teacher, Eraques. Also after these gliders (a time machine hover board) are invented, MX finds out they go from world to world, time to time and comes up with a plan to bring the keyblade war to the past to kill the ancestors before the birth of the now three heroes.
Eraques tells the three to go to the past and save people from the unversed (a futuristic being) especially the seven princesses of heart.
They learn of their ancestors through Master Yen Sid. Terra then goes to find Riku, hencing why he sees his ancestors future self through vision slowing forming into Ansem. Kairi goes to save Kairi from the unversed and puts a protective spell on her, while Ventus seeks Sora to stay with his heart, so he actually can stay by to protect him.