"Darkness can not be destroyed. It can only be channeled."
—Master Xehanort to Terra
Master Xehanort is the main antagonist of the eponymous "Xehanort Saga". A Keyblade Master who once taught Ventus, Master Xehanort believes that darkness and light should exist in perfect balance, and for this reason, he seeks to conquer Kingdom Hearts and recreate the worlds according to his wishes. Through his many different incarnations, including Terra-Xehanort, his Heartless, Ansem, Seeker of Darkness, and his Nobody, Xemnas, Xehanort is ultimately responsible for the series of events that repeatedly threaten the worlds with destruction, including the mass release of Heartless into the Realm of Light and the creation of the Nobodies and Organization XIII. He has also brought tragedy to many that have crossed his path, including Sora, Riku, Kairi, Terra, Ventus and Aqua.
Like the members of the original Organization XIII, Xehanort's name contains the letter "X", along with the anagrams "Another" and "No Heart".
In his true, original form, Master Xehanort first appeared in the teaser for Kingdom Hearts: Birth by Sleep as the antagonist battling Terra, Aqua and Ventus alongside Vanitas. Despite being the true main antagonist of the "Dark Seeker Saga", Master Xehanort did not appear until Birth by Sleep, the series' sixth installment, and did not meet Sora face to face until Dream Drop Distance, the seventh. This is because up until Kingdom Hearts II, it was believed that Terra-Xehanort, then simply known as "Xehanort" and the original persona of Ansem, Seeker of Darkness and Xemnas, was the true Xehanort and the series' antagonist, until Birth by Sleep revealed that the character was simply a possessed version of Terra. His origins and connection to Terra-Xehanort were hinted when it was revealed that he would be voiced by the late Chikao Ostuka, the father of Akio Otsuka, who voiced Terra-Xehanort (and Ansem).
In the English version of the series, Master Xehanort is voiced by the late Leonard Nimoy, who was chosen specifically by Tetsuya Nomura because of his role as Mr. Spock from Star Trek and because Nomura himself is a huge fan of both Star Trek and Star Wars, two franchises he intended to pit against each other by additionally casting Mark Hamill as Master Eraqus, Xehanort's enemy. Of note, Birth by Sleep was originally intended to be Nimoy's final performance before his retirement in 2010, but he has since come out of retirement and reprised the role in Dream Drop Distance.
It is also currently unknown who will replace Chikao Ostuka and Leonard Nimoy respectfully as Master Xehanort in the Japanese and English versions of Kingdom Hearts III.
Like Ansem and Xemnas, Xehanort is shown to be a cunning, callous, and pragmatic being with the sole goal of merging with Kingdom Hearts and obtain ultimate knowledge, the lives of innocent people and even those who care for him meaning absolutely nothing to him. To this end, he is willing to put whatever he pleases on the line or destroy anything in his way, demonstrated when he manipulated several characters such as Terra, Aqua, Ventus, Braig, and even Maleficent, created Vanitas, a sentient being born for the sole purpose of forwarding his evil plan, and slew his old friend Master Eraqus in cold blood after tricking Terra into defeating him in battle. He even shrugged off ruining Terra, Ventus, and Aqua's lives by telling Mickey and Riku that they were destined to fall, and went as far as to refer to Sora as a "dull, ordinary boy", despite Sora having foiled several of Master Xehanort's plans. His attitude towards his own allies is quite apathetic; he showed a lack of concern for Vanitas' misery from his separation and did nothing to help him until Vanitas was willing to obey, and had little to no regard for the killing of Eraqus. In the case of Sora, Xehanort tends to have a series of back-up plans to occur whenever his previous scheme fails. Yen Sid mentions this to Mickey while discussing Xehanort, and describes him as a brilliant strategist.
Despite his overweening malice, Xehanort has been shown to have some slight sympathy for his victims, such as when he takes pity on Ventus, following the rupture of his heart, and decides to leave him to perish peacefully on the Destiny Islands. All of Xehanort's other seeming virtues are revealed to have sinister intentions behind them, demonstrated when he only took Ventus as an apprentice and cared for him solely with the intention of making the boy his vessel once he was strong enough, and when he only accepted Master Eraqus's invitation to witness the Mark of Mastery Exam to win back Eraqus's trust following their falling-out and sabotage the exam to put Terra on the path to darkness so as to make him his new vessel after deciding to instead use Ventus for the χ-blade.
Overall, Xehanort is the true catalyst of all the events in the series so far, effectively making him the true villain of the Xehanort Saga, having been responsible (either directly or indirectly) for converting Terra in the first of his many vessels, arranging his younger self's quest through time, the Disney villains' hunt for Kingdom Hearts, the creation of Vanitas, the Unversed, and the original Organization XIII, and the release of the Emblem Heartless and Nobodies. Xehanort has a few unusual tendencies and quirks, however, such as turning away from the people he speaks to, or over-dramatizing his hand gestures when delivering a speech, even flexing his fingers in an unusual manner, as if itching to grab something.