Xehanort has been a prevalent antagonist throughout the entire Kingdom Hearts franchise. In one way or another, he's responsible for ALL of the bad that happens throughout the series.
With the release of the mobile title Kingdom Hearts Dark Road, players can witness Xehanort's fall into Darkness and what he was like beforehand. Dark Road is just the first piece of his story and the first step on his journey to becoming Kingdom Hearts' greatest villain.
Dark Road
Dark Road follows Xehanort as he leaves behind his home and follows the Keyblade wielders' path that came before. Before his departure, Xehanort lived a life much like his future rival Sora. He lives on Destiny Islands and spends his days dreaming of exploring beyond the bounds of his home. Reoccurring dreams plague him, detailing the events of the Keyblade War and its participants. It's immediately after one of these dreams that he's approached by a mysterious figure in a brown cloak offering him the chance he seeks to explore other worlds and leave his home behind. He doesn't know it at the time, but this figure is his own Heartless, Ansem, having traveled back through time to ensure the young Xehanort follows the path that will ultimately lead him to Darkness.
After awakening in Scala ad Caelum, Xehanort quickly begins to make friends as he goes through training to become a Keyblade wielder. Xehanort is the only one among his friends that has ever been to another world, and he's often the center of his friend's attention alongside Eraqus. Master Odin soon offers the group a task to investigate the disappearance of other Keyblade wielders. Xehanort's travels and experiences throughout this journey began to shape him, opening him up to the idea that Darkness is born from within the heart.
By the end of their adventure, only Xehanort and Eraqus survive. This loss, alongside the suspicion that Master Odin isn't telling them the whole truth, spurs Xehanort to continue exploring the worlds alone. Darkness eventually takes root in his heart. Sometime between his departure and return to take the Mark of Mastery exam, Xehanort is once again approached by the Heartless of his future self, Ansem. He is tasked with traveling throughout time and gathering 12 incarnations of himself that would ultimately form the true Organization XIII.
He loses these memories when he returns to his proper place in the timeline, but they imprint upon his heart and guide his future actions. Xehanort successfully passes the Mark of Mastery exam alongside Eraqus and is given free rein to travel the other worlds and take on pupils. During his travels, he encounters the Master of Masters and becomes disillusioned with what he calls the "false Light."
Birth By Sleep
After years of observing the worlds and learning more about the Keyblade Wars, Xehanort seeks to recreate the conflict and use Kingdom Hearts' power to rebalance the worlds. To accomplish this goal, Xehanort realizes he must reforge the χ-blade. Sensing great Light and Darkness within the boy, he takes on Ventus as an apprentice and splits the Darkness and Light inside him into two separate beings.
With his plan in place, Xehanort leaves Ventus' Light side in the care of Master Eraqus and awaits the day he's strong enough to clash against his dark self. After years of training alongside Terra, Aqua and Master Eraqus, Xehanort returns during the senior students' Mark of Mastery exams to inspire doubt in Terra's heart. While the trio of Keyblade wielders investigates the unversed at Eraqus' request, Xehanort seizes every opportunity to continue pushing Terra towards Darkness.
Xehanort's efforts culminate in a battle between the students, Xehanort and Ventus' dark counterpart. Ventus thwarts Xehanort from reforging the χ-blade, but in the process, Xehanort chooses Terra as his new vessel and possesses him. After an ensuing battle with Aqua, an amnesiac Xehanort is recovered by Ansem the Wise. Over time, Xehanort would encourage Ansem to conduct experiments to investigate the heart's true nature and create the emblem-clad Heartless.
Kingdom Hearts
Xehanort banishes Ansem to the realm of Darkness and claims his students and name for his own. He's responsible for Radiant Garden's fall and spiriting Kairi away to Destiny Islands while she's still a child. At this point, he's recovered the use of his Keyblade and uses it to take his own heart, splitting himself into a Heartless and Nobody. His Heartless seeks to open Kingdom Hearts by flooding the worlds with Heartless and gathering the seven Princesses of Heart; the Nobody would also aspire to open Kingdom Hearts by creating Organization XIII and manipulating Sora to defeat Heartless.
Sora and his allies would ultimately defeat Xehanort's Heartless and Nobody, causing the original Xehanort to regain his form. Now back in one piece, Xehanort revealed Organization XIII's true purpose was to provide 12 additional vessels for himself. The work that started when Xehanort was visited by his own heartless as a boy was nearing completion as Xehanort found his vessels during Sora and Riku's Mark of Mastery exam.
Xehanort successfully reforges the χ-blade after orchestrating a massive battle between the forces of Light and Darkness. It's only after he destroys Kairi's body in front of Sora that he forces the hero to attack him, completing the process. Sora and allies can only defeat Xehanort with the help of Master Eraqus, whose heart was residing within Terra since the day of his death at Terra and Xehanort's hands. Finally defeated at the hands of his old friend, Xehanort accepts his end. He shares a final moment with Eraqus before the two fade into the afterlife together.