Terra's calm and cool exterior hides a heart that would fight for his friends, oblivious to the fact that Master Xehanort is attempting to seduce him into darkness, he begins to doubt those closest to him, and only recognizes his mistakes after the fact.
He shares a strong bond with his partners, Aqua and Ventus.
He chooses Riku as his successor, making Riku destined to wield the Keyblade in the future.
He summons his Keyblade in a very unique way. The weapon is summoned like data, the numbers then replaced by small cubes.
Terra is hinted to be perhaps the strongest character in the series if he could reach his full potential. He's said to be a Keyblade Master in terms of power levels and just lacks the official title, and he proves that when he defeats two separate Keyblade Masters in single combat. Eraqus comments on his considerable strength at the start of Birth By Sleep, and the rest of the game sees Terra just get stronger and stronger.
His experiences with darkness bring a negative vibe to him, but Terra holds his bond with Ventus and Aqua in high esteem, willing to fight for them despite his ties to the darkness, and wishing to help them long after his possession. This is seen with both the new Xehanort and the Lingering Will, both instances of Terra's faded consciousness acting on Ventus's and Aqua's behalf.
He also holds Master Eraqus in high regard, viewing him as a surrogate father, which he proclaims to Xehanort. After destroying Master Eraqus in order to save Ventus, Terra mourns his loss greatly and sheds tears for his Master.
After years of Xehanort successfully taking over his body and never being able to recover it, it was nice to Terra, who is revealed to be Terra-Xehanort's Guardian, grab Xehanort's face and then finally reclaim his body with Sora's help, killing this incarnation of Xehanort entirely and finally ending the long nightmare, and reuniting with Ventus and Aqua.
Terra struggles to keep himself on the right path, and is constantly tempted by the power of darkness, similarly to Riku.
While not gullible, Terra can be overly trusting, which most villains use to their advantage, leading to several of his misadventures.
His English voice actor, Jason Dohring, didn't really put in much of a great performance in Birth by Sleep. Thankfully, he improved in the following games, especially III.
By continuing to use the darkness, Terra's heart becomes weakened. The straw that broke the camel's back was Xehanort orchestrating Terra's emotionally-supercharged duel with Eraqus, then killing him just as he's beaten by Terra. When Terra gives into his rage, unleashing the full extent of his inner darkness, Xehanort uses this as the perfect opportunity to take over Terra's body. Thus, Terra has become the one thing he's hated the most: Xehanort himself.
Despite that freeing him from Master Xehanort's control is one of the pivotal moments in Kingdom Hearts III, Terra doesn't actually get to do much or take part in the fight and is only briefly able to aid the Guardians of Light as the Lingering Will until the final battle.
His name means "Earth" in Latin, in Italian, in Portuguese and in Catalan, similar to Riku's name in Japanese meaning "land".
The official Japanese website refers to Terra as the "Vast Earth Protecting Bonds" (絆守る大地 Kizuna Mamoru Daichi?).