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.

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 and 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.