Not long after Sora was born, his heart connected with that of a boy named Ventus. Ventus's heart had been torn apart, but Sora saved him by sharing a part of his own.
Several years later, three young Keyblade wielders—Terra, Aqua, and Ventus—found themselves embroiled in a battle with Master Xehanort. In their travels, Master Aqua and Terra both met Sora and his best friend, Riku. Sora promised Aqua to keep Riku on the right path, while Riku made a secret promise with Terra. Not long after, Terra became a vessel for Master Xehanort, and Aqua vanished into the realm of darkness. As for Ventus, he shattered his own heart to protect his friends and returned to Sora's heart, where he sleeps to this day. Years later, after Sora got older, he unknowingly hid his friend Kairi's heart inside his own to protect it. When Sora became a Heartless, he also inadvertently gave form to Kairi's Nobody, Naminé, while creating his own Nobody, Roxas.
Another being was also created: a replica named Xion. Xion was formed from Sora's memories of Kairi, and she had the power to copy Roxas. Ultimately, Xion and Naminé realized their place and guided the pieces back where they belonged; Naminé returned to Kairi's heart, and Xion and Roxas returned to Sora's. Many hearts have touched Sora's, and those connections remain—but their stories end in sadness, a sadness in which they still sleep.