The story of Sora in Kingdom Hearts Birth By Sleep

Sora and Riku are shown as best friends and rivals as they grew up. They are often seen around the islands battling with wooden swords, racing, and doing anything else that is competitive.

Some time prior to Kingdom Hearts Birth by Sleep, Sora’s newly born heart encounters the injured heart of Ventus and mends it, offering to help him restore his broken heart until help is not needed. This allows them to form a connection with one another, and causes Vanitas’s face to resemble Sora’s.

By the time Sora is four years old, he encounters Terra and Aqua, two Keyblade wielders of the generation before him, on separate occasions. He never spoke with Terra, but they have seen each other on Destiny Islands. Terra shows no interest in Sora because he is unable to sense his potential, and is instead attracted to Riku. Aqua recognizes their friendship as hers to Terra and Ventus and tells Sora to protect Riku if he were to fall into the hands of darkness, which he agrees. Ventus had visited the islands multiple times, but had never met Sora face to face.
After the credits of Kingdom Hearts Birth by Sleep, Sora and Riku rest on a beach. When they prepare to go home, Sora suddenly feels sad. Riku tells him that perhaps someone from another world connected to him is sad somewhere and tells him to try to reach him. Sora closes his eyes and reaches out, which results in him being sent to the Awakening and encountering Ventus’s lost heart. It had lost its path and must return to slumber, though it is reluctant to do so and asks if it could become a part of Sora’s heart. Sora accepts the request saying that as long as he can make it happy by doing so, he is willing to.