Terra

Terra (テラ, Tera?) is an original character created by Tetsuya Nomura. He first appears as one of the main protagonists of Birth by Sleep, originally wielding the Earth Shaker Keyblade. He is voiced by Ryōtarō Okiayu in the Japanese version of the game. His name means "earth", from Latin, both it and his Keyblade follow the "land" theme Riku represents. Though he takes the Master Qualification Exam alongside Aqua, Terra is unable to become a Keyblade Master. Subsequently, he is sent by Master Eraqus to seek out Master Xehanort and his cohort when they both disappear. Terra is believed to have succumbed to darkness for power because of Master Xehanort, believing he had done something to his heart. In the end of the game, Master Xehanort's heart possesses Terra's body and mind, transforming him into the current Xehanort complete with grey hair, while the will of his own heart finds residence within his discarded armor and cape, wielding Gaia Bane Keyblade and becomes known as the Lingering Sentiment (留まりし思念, Todomarishi Shinen?). The Lingering Sentiment engages the new Xehanort in a fierce battle, and wins, but only succeeds in knocking Xehanort out long enough for him to be transported to the Radiant Garden, where he confronts Aqua and faces amnesia.

The Lingering Sentiment later appears as an optional boss in Kingdom Hearts II: Final Mix. When Sora meets the Sentiment in the Keyblade Graveyard, he calls out Xehanort's name and attacks Sora, who manages to defeat him after an intense battle. The Sentiment respectfully acknowledges Sora as a Keyblade Master after this. If Sora comes back to challenge the Sentiment again, he states he has nothing else to give him, and that the only thing he has left is his hatred towards Xehanort. Prior to Birth by Sleep, Nomura had stated the connection between Terra and the Lingering Sentiment battle was at the time uncertain and irrelevant.

The climax of Kingdom Hearts: Birth by Sleep shows that Terra, having Eraqus's heart, and Master Xehanort are still battling for dominance over the new Xehanort's body. What with the apparent destruction of both Xehanort's Heartless and Xemnas, the status of this internal battle is unknown, but it is known that Terra, is some shape or form, is still alive, as Sora sets out to rescue him, Aqua and Ven at the climax of Birth by Sleep.