Xehanort was born on Destiny Islands and believed that his world was too small. Xehanort was encountered by his future Heartless to find 13 vessels in the future. When he returned to the present, Xehanort fled to the Land of Departure. Xehanort was wearing Keyblade armor. Over time, Xehanort was interested in the Keyblade War. The war began many years ago where people wanted the light of Kingdom Hearts. The light was guarded by the X-Blade. Keyblades were made after the X-Blade. The clash shattered the X-Blade into 20 shards, seven of light and thirteen of darkness. Xehanort used a black robe to accept darkness instead of destroying it. He is a rival with his classmate, Eraqus.
Xehanort found a boy named Ventus and wanted who he wanted as his vessel. The attempt failed and Ventus' darkness was extracted by Xehanort's keyblade. The darkness became a person named Vanitas. Xehanort took Ventus to his birthplace so that he could replenish from his injuries. Ventus was healed by a new born's heart which filled the broken part of him that previously held his darkness. Ventus ended up having amnesia. Xehanort took Ventus to the Land of Departure where Terra and Aqua were training to become keyblade masters. Xehanort decided that Terra should be his vessel instead of Ven. 4 years later, Terra and Aqua were examined for the Mark of Mastery. Terra failed since Xehanort used darkness on him. Xehanort informed Terra that he is truly worthy of the title that is master.
Xehanort met Maleficent and made an alliance with her. Maleficent was tasked to find the seven princesses of heart while Xehanort could make 13 vessels of darkness. Xehanort lured Terra to Radiant Garden where his friend Braig was. After Braig was scarred by Terra's darkness, he ended up trusting Xehanort and Xehanort gave part of his heart to Braig. Xehanort came to Ventus and told him to merge with Vanitas to forge the X-Blade. Xehanort told Terra to rescue Ven from Eraqus. Xehanort came and killed Eraqus after his battle with Terra and destroyed the Land of Departure. Xehanort went to the Keyblade Graveyard where the Keyblade War took place.
Xehanort told Terra what he had done to Ven. Xehanort saw the X-Blade being forged and stabbed himself with his keyblade to use his heart and possess Terra. Xehanort's heart went inside Terra's body. Terra's will was so powerful that it animated his keyblade armor which battled Xehanort to regain his body, until the X-Blade was destroyed. Xehanort came to Radiant Garden confused as to who he was. He battled Aqua until his guardian sent him the realm of Darkness. Aqua dived in to save her friend who Xehanort possessed, sending Xehanort, her keyblade, and armor back to Radiant Garden. Ansem the Wise found Xehanort and decided to make him a pupil. Braig wondered if Xehanort was Terra since it's Terra's physical body.
Xehanort used experiments on Heartless until Ansem banned him from experimenting any more. Xehanort studied Ansem's work and banished him. After working on experiments, he stabbed Braig and declared that he is not Xehanort, but Ansem. Over the next ten years, Ansem (Xehanort's Heartless) and Xemnas (Xehanort's Nobody) each did a half of Xehanort's task. Ansem and Maleficent were tasked to find the 7 Princesses of Heart and open the door to Kingdom Hearts while Xemnas was tasked to find 13 nobodies, hook them up to Kingdom Hearts (One made of the hearts of people), and fill them all with Xehanort's heart and mind. However, through weakness of body, will or trust, most of the members of the Organization were inadequate. Therefore, naturally they never had a chance to attain their goal. Even that was to be expected.
As a back up plan, Ansem wearing a cloak time travelled to Xehanort when he was young to find 12 versions of himself in the future. Ansem waited on Destiny Islands until his encounter with Sora and the destruction of the world. Both Ansem and Xemnas were destroyed by Sora. Both deaths led to the return of Xehanort. Xehanort captured Sora and revealed his plans to Riku and Mickey. Xehanort and Organization XIII retreated for the final battle.
While living on Destiny Islands, Xehanort was visited by Ansem the seeker of darkness and given the power to travel through time. He was also given a task to travel to the future and gather 13 versions of himself to either clash with the 7 Guardians of Light or capture the 7 Princesses of Heart, in order to forge the X-Blade. During his time travelling, he runs into Aqua and battles her using his keyblade transformation which causes his keyblade to look like Ethereal Blades.
After many years, Young Xehanort along with other versions of himself, take over Sora and Riku's Mark of Mastery exam so as to capture and turn Sora into one of the 13 Seekers of Darkness. Riku is determined to save Sora but runs into Young Xehanort and the other Seekers of Darkness, who prevent him from interfering with their plan. Mickey appears with a flash of light and casts Stopza on everyone other than Riku in order to grab Sora and leave. The time spell doesn't work on Young Xehanort (due to his powers over time) who knocks Mickey unconscious and fights Riku. After the battle is over, Lea, Donald, and Goofy arrive just in time to avert the gathering of the 13 Seekers of Darkness, causing the different versions of Xehanort to go back to their original times and forget what had happened. Though they travelled back to their own times, the destinies and time to again, gather the 13 is nigh.
Notable Abilities (Young Xehanort)
Young Xehanort has the power over space time. Using this power he is able to reverse time during fights (preventing himself from being knocked over or even to the very start of the battle), stop time completely whenever he wishes, slow down his opponent, appear anywhere on the battlefield, create multiple versions of himself, turn invisible, send his slashes towards his enemy, and take away his opponent's abilities.
He is a very powerful Keyblade Wielder who is able to use Keyblade transformations, turning his Keyblade into dual wielding blades which resemble Ethereal Blades - holding them backwards or connecting them so that they have the appearance of a double-bladed lightsaber. Additionally, he able to transform it into a massive whip to attack opponents from afar (he can also incorporate his power over space time with this transformation in two ways; creating orbs which each shoot out his whip simultaneously, and bending the space around his whip so that it surrounds his enemy and attacks from unpredictable angles) and hook onto the target so that he may drag them towards himself.
Being a Keyblade Wielder, naturally, he is able to use magic. Some prominent magic of his includes; surrounding himself in flames, summoning meteors, and high tiered magic which blinds his opponent with a flash of light while being cast (fire and/or ice).