Ventus or Ven, is a young boy who was originally Master Xehanort's apprentice. He was forced by Xehanort to battle Neoshadow heartless but was too weak to use his rage. Xehanort decided to extract Ven's darkness to create a being of pure darkness named Vanitas. Ventus whose heart was now broken, was left on Destiny Islands.There, he was greeted by a newborn's heart. The newborn wanted to help heal Ven's broken heart by fusing his light with Ven's. Ven's heart was healed and therefore, he became a being of pure light, no longer broken.
Ven moved to the Land of Departure where Master Eraqus was teaching Terra and Aqua in the ways of a keyblade master. Ventus, who no longer has his memories of Master Xehanort, began training under Master Eraqus. Ven, Terra, and Aqua became very close friends and all shared the same goal, to become keyblade masters. Shortly after Terra and Aqua's mark of mastery, Vanitas appeared before Ven, telling him that Terra was leaving him behind and will be a different person by the time he catches up. This caused Ven to chase after Terra despite Master Eraqus' wishes.
Xehanort's plan was to have Ventus fuse with Vanitas and forge the all-powerful X-Blade. When Ven came back to Eraqus, Eraqus was set on destroying Ven to prevent the forging of the X-Blade. Terra defended Ven who threw him into a Corridor of Light which transported him to Destiny Islands. There, he ran into Vanitas who threatened to kill Terra and Aqua if Ventus refused to forge the X-Blade.
At the Keyblade Graveyard, Ven came to face his darkness, Vanitas. Xehanort froze Ven and told Vanitas to kill Aqua. Vanitas knocked Aqua unconscious and before he was able to deal the killing move, Ven broke free of the ice. Ven and Vanitas ended up fighting, which was exactly what Xehanort wanted. Vanitas, revealing his face, looked like Sora since Sora's heart connected to Ven's in order to no longer be broken. Vanitas then fused with Ven, becoming whole and forging the X-Blade. Ventus had an internal battle with Vanitas while Aqua had to fight the Vanitas/Ven fusion who was wielding the X-Blade. Both of them won the fight, destroying the X-Blade and with that, Ven's life.
Ven's heart came to Sora and took refuge in his body while Aqua returned to the Land of Departure and transformed it into Castle Oblivion, in order to keep Ventus' body there in a hidden room.
After Sora stabbed himself with Ansem's keyblade, Roxas ended up looking and sounding like Ventus.