So... it seems your hearts have led you to obliteration. Perhaps it doesn't pay to be too loyal to one's heart. I will have to be sure and remember that.
Being the founder of Organization XIII and apparently the oldest of Nobodies, Xemnas is truly emotionless in every sense of the word and does not even remember what it was like to have any feelings. He claims that this allows him to have a clear understanding of the heart, but in reality, this has warped his mind into believing that only anger, hatred, and other negative emotions are what make up most of a heart.
His lack of emotion deprives him of remorse over his actions, as well as the loss of the other members of the Organization. The only thing he truly cares about is Kingdom Hearts and his desire for power, and even laments it when it is damaged. However, on some occasions, he has appeared to value friendship, as he is shown to converse with the armor of Aqua in the Chamber of Repose, although this is likely due to the influence of Terra as he is in a sense, Terra's body.
Unlike the other members of the Organization, Xemnas does not seem to want Kingdom Hearts for a true existence. Rather, much like his Heartless counterpart, he wants it to use it to obtain power and remake the universe in his image. This concept is furthered as Xemnas thinks that nothingness is the source of much power and does not seem to despise it in the slightest. This goal stems from that of one of his two original personas, Master Xehanort, whom also desired to unlock Kingdom Hearts for the same purpose in Kingdom Hearts Birth by Sleep.
In other matters Xemnas speaks with calmness and deep authority, strangely similar to Master Eraqus. He has a tendency to be overly melodramatic making long-winded speeches, exaggerating his false rage and sorrow with wide sweeping arm movements and hand gestures, like many villains. While ridiculous in nature, his exaggerations are representations of his lack of emotion. He's very intelligent overall, and knows how to manipulate others, particularly Sora and his friends for his own needs.
Xemnas's personality seems to share traits from both beings who composed his original persona, Master Xehanort and Terra. From the former he inherited his manipulative nature and the ideas for the black coat and adding the letter "X" to rename himself and his fellow Organization members.[2] It is very hard to say how much he inherited from Terra, but there is at least the familiarity with Aqua's armor, to which he refers to as "friend", and his strong desire to find his other "friend" in Castle Oblivion, as he has had SaΓ―x find the Chamber he sleeps in. Another example is when Roxas falls into a comatose, saying "So, sleep has taken you again."
s the leader of Organization XIII, Xemnas wields nothingness, and manifests this as blades of energy called Ethereal Blades. In his encounter with Sora as the "Unknown" in Kingdom Hearts Final Mix, and their first battle in Kingdom Hearts II, Xemnas is very agile, and attacks with swift acrobatic combos with his blades. He can also teleport, fly, and create energy barriers to shield himself. Besides using them as melee weapons, Xemnas can fire his Ethereal Blades as laser-like projectiles, and can either fire them directly from his hands or create them in mid-air. In all three battles with Xemnas's unarmored form, Xemnas also incorporates some sort of attack that drains his opponent's health with a sphere of dark energy, though the exact manner of this has varied (see below).
In Kingdom Hearts Final Mix, in his encounter as the Unknown, Xemnas also had the ability to encase himself in a blue ball of energy that made it impossible for Sora to attack him, and would also fire out a set of powerful lasers. The ball would follow Sora around if he ever to tried to get away from it. In his first battle with Sora in Kingdom Hearts II, Xemnas can encase him in a field of dark energy that steadily drains his health, that can only be broken by using the reaction command. In the final battle in Kingdom Hearts II, Xemnas uses an attack known as the "Invitation to Nothingness", in which he ensnares opponents in a sphere of dark energy that slowly drains their life force.
In Kingdom Hearts 358/2 Days, Xemnas has a limit break named All-Vanity. During All-Vanity, Xemnas summons a barrier around himself, and hovers around the field. Any enemies that touch Xemnas's barrier will take damage, and cannot deal damage to Xemnas. During Xemnas's Final Limit, Xemnas creates two large powerful lasers from his hands that he fires around himself at a long range. Any enemies that are hit take heavy damage.
While in his armor and aboard the World of Nothingness, Xemnas has access to the powers and weapons of his fallen Organization comrades, such as Xaldin's lances and Demyx's power over water. In this form he remains in his throne, attacking enemies with his massive sword when they come close. He can still project barriers, and summon crests of light in the air to fire white beams of energy at his enemies.
In the final battle of Kingdom Hearts II, Xemnas's power increases dramatically. He can create copies of himself, summon torrents of white and black thorns much like Twilight Thorn, and retains his normal ability to use his Ethereal Blades and project barriers. As his ultimate, final attack, Xemnas summons thousands of Ethereal Blades around Sora and Riku to bombard them from all sides with no hope of escape.
Xemnas uses weapons called Ethereal Blades[3] (γ¨γ’γͺγ’γ«γγ¬γΌγ Eariaru BurΔdo?, lit. "Aerial Blades"). They are beams of red energy and extend directly from the palms of his hands, without any hilt or handle. He can summon or disperse them at will, and is able to change their length as well. They are solid despite their appearance, and, despite seemingly lacking a cutting edge, are wielded like swords. The Ethereal Blades can also be used as projectiles and can suspend themselves in midair around a target before firing at them. As a desperation move, Xemnas can surround his enemy in darkness and fire an amazing amount of Ethereal Blades at them.
The names of Xemnas's various Ethereal Blades seem to be references to iron-fisted leadership. Perhaps due to his preeminent power among the Organization, only one of his Ethereal Blades, Infinity, actually grants him a support ability.
Xemnas's origins as Terra and Master Xehanort indicate that he may have been capable of wielding a Keyblade, but just preferred not to. [4] His use of Master Xehanort's Keyblade Armor during his battles with Sora and Riku further supports this. However, this implication seems to contradict Xemnas's reasons for using Roxas and Xion to further his plans.
The Superiour of Organization XIII, his past to him, means almost nothing.
(Most of this is copy and pasted from Wikia. Well. All of it.)