Terra

Originally called the "Armored Youth", the "Enigmatic Soldier" or the "Enigmatic Knight", Terra is the tallest and assumed to be the oldest of the three keyblade knights. The three are portrayed as the protagonists against the old man and his companion, and he, like the others, wields a keyblade. He begins the battle by rushing at Master Xehanort, but the old man avoids him by separating himself from the others with a makeshift cliff. Terra is hit by a blast of lightning from the old man's guardian, but ignores him to pursue Master Xehanort. He is at first unable to reach the man because of the cyclone of keyblades, but after being aided by Aqua's magic, he is able to reach the man's cliff and attack him. The old man, wielding a keyblade of his own, is able to fend him off with a freezing spell. Once the old man gets a hold of Ven, a younger knight, Terra is thrown off the cliff by the keyblade cyclone, ending his involvement in the struggle. He is seen one last time after the appearance of Kingdom Hearts, when he pulls off his helmet to gaze up at it. His face and his reaction to the appearance of Kingdom Hearts, which turns his eyes yellow, have given rise to much speculation among fans, who have connected him to Xemnas/Xehanort, Saïx and other characters, though no concrete connection has been made by Nomura. In early promos to "KH:BBS", Terra was searching for Master Xehanort, only to have feelings of being betrayed when he finally finds him.

In Kingdom Hearts: Birth by Sleep, Terra will be featured as a player character. He also has been revealed to know Fire magic. He is also known to have some as-yet-unknown connection with Maleficent.

As a Boss
Terra did appear in Kingdom Hearts II: Final Mix as a secret boss, seeing Sora's similarities to Ven before attacking him. Unlike Sephiroth, another secret boss in the game, Terra fights against both Sora and his main companions, Donald and Goofy. After losing once, he can be challenged again, and mentions both Aqua and Ven before mentioning his harboring of a deep hatred for "Xehanort". It's unclear why he attacks Sora, but when talking he appears to go berserk and mistake him for Xehanort. After the fight, he acknowledges Sora as a Keyblade master.

He, like the game's other secret bosses, is difficult to defeat; his most basic moves are attacking with his keyblade, and can fly at very high speeds. His main abilities are transforming his keyblade into various attacking forms. He can turn into a whip that does less damage then the blade form, but its greater speed and reach makes up for it (it's so fast that if Sora is hit by it he won't be able to dodge or block the other hits), or a massive cannon (which is even bigger then he is) that fires out a large, very powerful ball of energy (Sora, however, can deflect this back to Terra, which leaves him very vulnerable), a jet-ski like object that lets him fly extremely fast, a cog-like object that spins around so fast that if Sora gets hit by it once, then the rest of the hits will also hit Sora. He can also conjure a "ritual" like attack, which undergoes into a berserker-like condition (similar to Riku's transformation into Dark-mode) in this form, red auras surrounds him, which he can summon another Keyblade. This, when added with his combos, makes him extremely lethal.

In addition to his keyblade, Terra has other abilities as well. He can also create a unusual looking seal that either creates a blue orb or an orange orb. An orange orb causes Sora to be unable to attack and a blue orb prevents him from using magic or items. He can also conjure drone-like objects which fire plasma at Sora and his party. A particularly deadly attack is trapping Sora in a triangle of objects that drain his heath at a very fast rate (similar to an attack used by Xemnas, but much more powerful), and only a "Break Free" command on Sora command list can save him from it. Finally, Terra has one last attack where he gathers energy and unleashes it in a flurry of strikes that reduce Sora to 1 hp. Interestingly, when he strikes Sora with his Keyblade(s), most of his combos resembles Sora's combos, most notably Valor mode's.

Terra is the most powerful enemy in Kingdom Hearts II: Final Mix and is arguably the hardest boss in the series. His level of power compared to all other bosses in the series may be a foreshadowing of sorts to something as-of-yet unrevealed.

Secret Trailer Summary
In the opening of the secret trailer, we see an unknown man in armor standing on top of an object. Terra seems to be standing triumphantly, holding his massive keyblade behind his back. He then notices something from off screen and jumps down. As he walks away, the object he was standing on blows away, like it was dust. As Terra walks to the middle of the crossroads, Aqua appears from another direction, along with Ven appearing in the opposite. Three keyblades lie in the center: Kingdom Key, Way To Dawn and Mickey’s Keyblade. Terra picks up the Kingdom Key. Aqua picks up Mickey's Keyblade. And Ven picks up Way To Dawn. Then they all notice a shadowed figure off in the distance.

The three knights face down the mysterious figure off in the distance. As the figure draws closer, his face becomes visible. The man is an Unknown Elder Mage (U.E.M.), who then splits himself into two beings. By doing this he creates the Dark Soldier (D.S.). Ven starts to move towards them, only to be halted by Terra who goes after them himself. As he dashes towards them, the U.E.M. casts a spell that lifts a large wall of earth to block his path. He then lifts himself and the D.S., up with another wall from the ground. As Terra watches them rise up, the D.S. jumps down and casts a powerful thunder spell on him. This leaves his armor badly damaged, engulfed with small flames. Ven then runs after the D.S., with Terra soon following him. The U.E.M. then casts another spell, where many keyblades burst from the ground and attack in waves. Ven continues his chase, while the D.S. casts blizzard spells to keep him off of his trail.

Aqua tries to keep up with them, but then notices the wave of keyblades. The D.S. jumps on top of them and rides on top of them, casting more blizzard spells as the two knights try running away. He has the upper-hand while he rides on top of the wave of keyblades. They manage to strike Ven, but he quickly regains balance. When the D.S. jumps down, Ven tries to get a hit in but is simple knocked away. Aqua uses this time to cast a thunder spell, but the D.S. avoids it and rides off on the keyblades again. Terra tries to run from the wave of keyblades, since he was targeted next, but accidentally falls off a cliff in the process. He attempts to get a grasp by stabbing his keyblade in the wall, however the wave of keyblades blows off the part of the cliff where he was on.

Aqua notices that Terra is plumeting. With her distracted, another wave of keyblades strike her from the ground below. The force lifts her up into the air and then she endures a hard impact as she is brought back down. Aqua's helmet is obliterated from the wave and her face is revealed. Ven runs to check on her and she is shown to be alright. She quickly casts reflect on Terra upon her harsh landing, seeing that he to is being assaulted by the wave of keyblades. Reflect reaches him and forces the blades to stay away from his body. Yet another wave of key blades assault him, but Terra remains safe with reflect still casted. The blades push the him up to the U.E.M., then the shield protecting Terra explodes. Aqua stays behind as Ven goes after the two threats.

Terra falls, and lands, not to far from the U.E.M. He stumbles trying to get on his feet, then begins to run towards the enemy. The U.E.M. strikes back at him with his own keyblade, managing to knock the Terra back and freeze his entire left arm. Ven attempts to strike the U.E.M. while he is distracted, but he just teleports behind him and grabs a hold of his head. Terra tries to run to help his friend, but is hit by another wave of keyblades. He is blown off the cliff once more. The U.E.M. tightens his grip, crushing the Ven's helmet. Aqua rises up, finally recovering from the attack earlier, and sees Ven in the hand of the U.E.M. The D.S. reappears and stands with his ally. A piece of the Ven's helmet breaks because of the tight grip, revealing his right eye. Aqua and Ven look at each other in horror as his is frozen alive.

The U.E.M. freezes his entire body and heartlessly throws him off the cliff. Pieces of his keyblade and suit break off as he falls to the ground. Before he touches ground, Aqua runs to him, barely managing to get to him in time. She holds his body and looks into his face. Vens eyes are still moving, showing that he is still alive. The U.E.M.'s keyblade then vanishes, being replaced with a glowing orb. The U.E.M. then launches it up into the sky, which suddenly opens up revealing a heart-shaped moon. Aqua clutches Ven's frozen body as she looks on in awe at the sky. We then see Terra, at the bottem of the cliff amongst the rubble. He finds the strength to get to his feet and remove his mask. He witnesses the heart shaped moon in. As Terra looks towards it, the screen violently zooms in on him. His blue eyes suddenly turn hazel.

Mickey appears on the edge of a hill, quite a ways from the battle. The mouse watches as the heart shaped moon breaks through the clouds. Mickey dons a new black outfit and is wielding the Star Seeker Keyblade. Mickey doesn't seemed please with the battle he saw, then the trailer ends.