Friendship Quote Vacation
That ‘ball of light’ is Ventus’s heart. His body lost its ability to connect with his heart when Vanitas shoved him into the realm of darkness to battle for the Kaiblade.
Remember that Kingdom Hearts lives on in the hearts of children, now that it’s locked away. So children’s hearts have exceptional powers. Sora healed Ventus’s heart in the beginning of the game (when Master Xehanort ripped all the darkness from his then-apprentice to try and forge the Kaiblade, the truly ultimate weapon of Kingdom Hearts), and then helped him to heal again by providing him with shelter for close to ten years.