Well it's not entirely clear about that. Then again, Sora's name means "sky" simular to Ventus as well as Riku and kairi's with Terra and Aqua so they each may be related to one another like parents or relatives, just a guess.
Birth by Sleep may shed some light on this. Right now i bought an imported Pokemon Soul Silver for DS and i'm loving the game even though my Japanese reading skills are poor.
i found the trailer for Birth by Sleep, it looks so badass
http://www.gametrailers.com/video/player/57937