"In your hand, take this key. So long as you have the makings, then through this simple act of taking, its wielder you shall one day be. And you will find me, friend. No ocean will contain you then. No more borders around, or below, or above. So long as you champion the ones you love." -- Terra, when he lets child Riku hold his keyblade.
Terra and Aqua train under Eraqus. One day, Xehanort dropped off Ventus. Ventus came to meet the two. Ventus is suffering from amnesia. Four years later, Terra and Aqua are chosen for the Mark of Mastery exam. Terra failed since Xehanort gave him darkness. As he travels from world to world, he is manipulated by Master Xehanort into unlocking the darkness within his heart. Terra was tricked into unlocking Aurora's heart so Xehanort can remake the X-blade. Terra and his friends found Xehanort and Vanitas at the Keyblade Graveyard. When Xehanort's heart was directed at Terra for a vessel (Ven was originally chosen), Terra's armor and mind resisted. He sets out to find Aqua and Ven. When Sora read Mickey's letter, it revealed the origins of Terra, Aqua and Ven. Sora vowed to find the three.
After eleven years becoming sealed in his Keyblade Armor, he came across Sora, Donald and Goofy. He first mentions Aqua and Ven and mistakes Sora as "the one he has chosen" (believing him to be Riku, who Terra chose to be the Keyblade wielder, but gave into Darkness and Sora was chosen instead). He then wonders if Sora is Master Xehanort, then attacks. After he is defeated, he acknowledges Sora as a Keyblade Master.