Xehanort Cheirismos, also known as Master Xehanort Cheirismos is a Keyblade Master, the first master of Ventus, and the creator and master of Vanitas, the primary original incarnation of Ansem and Xemnas, and the future self of Young Xehanort. He was the primary force that Sora and his comrades fought during their adventures.
Much like Xehanort, "Ansem", and Xemnas, Master Xehanort is shown to be a cunning, callous, and pragmatic being with the sole goal of merging with Kingdom Hearts and become all knowing, the lives of innocent people and even his closest friends 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; creates Vanitas, a sentient being for the sole purpose of forwarding his evil plan; and slays his old friend, Master Eraqus, in cold blood after tricking Terra into defeating him in battle.
Furthermore, when Riku and Mickey confronted him over ruining Terra, Ventus, and Aqua's lives, Xehanort simply told them that it was their destiny to fall, and dismissed Sora as a "dull ordinary boy" even though Sora had successfully thwarted several of his plans. In the case of the latter, 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 shown to have some slight sympathy, though the only known occurrence of this trait is when he takes pity on Ventus, following the rupture of his heart, and decides to leave him to die a peaceful death on Destiny Islands. All of Master Xehanort's other positive points 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, left him in Eraqus's care to simply make him stronger to be joined with Vanitas years later, and only accepted Master Eraqus's invitation to witness the Mark of Mastery Exam to sabotage it and 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.
Despite his sinister actions throughout the series, the root of Xehanort's malice appears to originate from his curiosity of how the universe was made and his lust to become all knowing. It is this trait that enticed Xehanort to leave the Destiny Islands in search of other worlds, ultimately starting his life as a Keyblade Wielder. This trait is shared throughout his various incarnations as Xehanort, Xehanort's Heartless, and Xemnas all justify their actions by believing that it is all for the sake of research.
His descent into darkness is owed partially to this lust as he wished to learn everything there is to about the worlds, including the Dark Realm. He does not believe in mindless violence or destruction without cause or motive, nor is sadistic in nature, shown by his merciful sympathy towards Ventus after rupturing his heart, but he will willingly engage in violence, cause destruction, and even commit cold-blooded murder if it benefits his quest for knowledge. Ultimately, knowledge and power are the only things Xehanort finds value in and only sees other living beings as tools to be manipulated if they are useful or destroyed if they are hindrances to his grand designs, but always with a greater purpose in mind.
Additionally, Xehanort's reports show the man in a different light than what is initially portrayed as an individual that does not cause destruction and tragedy for the sake of it, but to gleam answers from it. Unlike villains such as Maleficent, Xehanort does not seek to form Kingdom Hearts to purely obtain power, but to incite a Keyblade War and see firsthand just how the world was made from such a conflict. Through this observation, he would be able to become all knowing and create a universe that is more to his liking where light and darkness are more balanced.
While these motives of seeking knowledge and creating a more balanced existence were not initially evil in nature, they allowed Xehanort to feel no remorse for his actions and commit heinous atrocities throughout the worlds for years on end, turning Xehanort into a violent and amoral villain who is eventually consumed by his lust for knowledge, power, and darkness. Though Xehanort's goal of completing Kingdom Hearts is for various reasons, he is not above of being memorized by the absolute power that he would be granted through possessing the X-Blade and unlocking Kingdom Hearts; this lust and obsession would continue on with his Heartless and Nobody counterparts.
Regardless of his justifications and motives for his plans, Xehanort has shown an extreme disregard for all life. This disregard for life is seen in his ultimate goal of inciting another Keyblade War to simply quell his curiosity and remake the universe, which would cost the lives of billions of innocent people and the destruction of all worlds. He gleefully took the life of Master Eraqus, his former best friend, to simply make the darkness in Terra's heart stronger, showing that Xehanort's obsession with Kingdom Hearts overshadows even that of the affection he once felt towards a man he saw as a brother.
His counterparts also share in his extreme disregard with his Heartless destroying thousands of worlds to form a Kingdom Hearts made of the darkness from their destroyed remnants and unleash it onto the worlds while his Nobody created millions upon millions of heartless from the hearts of countless individuals and manipulated his own associates for his own ends, feeling no attachment to them whatsoever and deceiving their own emotions.
This trait is also very indicative of Xehanort's overall persona as he believes that only the emotions of anger and hatred within the heart give one power as they feed the darkness within all hearts. Xehanort's complete disregard for human life, lack of any empathy or remorse, and disdain of any emotion within the heart aside from anger and hatred arguably makes him a psychopath, unable to feel any attachment to anything aside from Kingdom Hearts.
While possessing amazing skills, stamina, and powers, even at such an old age, Xehanort rarely, if ever, brags about his skills and even accepts the fact that he is capable of being defeated. Knowing that not even he is unbeatable, Xehanort is always laying out numerous back up plans in case his original designs do not go as planned. This not only allows Xehanort multiple paths to take even if his schemes are spoiled, but keeps him one step ahead of all his adversaries, even the immensely wise and powerful Yen Sid. His modest demeanor and acceptance of possible defeat also contributes to his calm appearance as he knows that, even in defeat, he still has other alternatives to turn to.
After taking over Terra's body, the villain retains all of his personality traits as the new incarnation of Xehanort (before the latter's fight against the Lingering Will and Aqua) and "Ansem", who's attire is the same as Master Xehanorts'. Xemnas, meanwhile, appears to be influenced several times by Terra's characteristics, such as his goal of locating Ventus and speaking to Aqua's armor as a fallen ally and friend rather than an enemy. Regardless, Master Xehanort's influence appears to be the primary and dominant consciousness controlling Terra's body and mind as all his forms are obsessed with Kingdom Hearts and completing the alternative plans that Master Xehanort had laid out prior to taking over Terra.
Lastly, Master Xehanort also has a few unusual tendencies and quirks as well, 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. These actions seem very indicative of his obsession and excitement over his plans and visions, nearly losing his calm composure when discussing over what can be achieved and when fulfilling the most essential steps of his elaborate plots and calculated betrayals.
Overall, Master Xehanort is the true catalyst, either direct or indirect, of all the events in the series so far, such as the creation of all his counterparts, Vanitas, the Unversed, the Emblem Heartless, and Nobodies, Organization XIII and others hunt for Kingdom Hearts. He appears to be the most evil and dangerous of all of Xehanort's forms and where all their malice and evil truly stems from. What started as simple curiosity turned into pure malice and deception, turning the man into a massive threat and villain throughout all the worlds that has lived on through countless incarnations.