Young Xehanort is a time-traveling younger version of Master Xehanort who appears in Kingdom Hearts Birth by Sleep, Kingdom Hearts Re:coded, Kingdom Hearts 3D: Dream Drop Distance, and Kingdom Hearts III. He acquired the ability from his future self's Heartless, and he used it to manipulate events so that his future self's plans would come to fruition. He is a member of the real Organization XIII.
Before Kingdom Hearts Birth by Sleep
Kingdom Hearts Birth by Sleep
When Terra, Ventus, and Aqua travel to the remains of the Land of Departure after its destruction, they meet the time-displaced Young Xehanort, who hides his identity by keeping his hood up. After he is defeated, he departs again through time.
Kingdom Hearts coded
After the original Organization XIII has been defeated Young Xehanort travels to Radiant Garden, finds most of its original members unconscious. He wakes Braig and the two discuss Master Xehanort's plans and the details of his Keyblade for a short period. They then agree to take Isa, as Saïx, with them.
Kingdom Hearts 3D: Dream Drop Distance
The time-displaced Young Xehanort successfully gathers other incarnations of himself from across time, until they total twelve, including himself and the elder Master Xehanort, and is provided with a replica body for his heart when he reaches the present.
At the beginning of Sora and Riku's Mark of Mastery exam, the disembodied Ansem appears, in his robed appearance. He causes Sora to be marked with the Recusant's Sigil, allowing him to be tracked and guided through the Sleeping Worlds by Young Xehanort and his comrades.
He appears before Sora and Riku in several worlds, often accompanied by either Ansem or Xemnas. They taunt the two, Sora about his unknown links to others' hearts, Riku about his dark past, and both about being trapped in their dreams. Young Xehanort chooses to make Sora the thirteenth vessel for Master Xehanort's heart, as Riku had developed a resistance to darkness. In the Symphony of Sorcery, Xehanort confronts Sora, commenting on the idyllic and beautiful dream-like landscape. Xehanort reveals he is not part of Sora's dream and ominously tells Sora to keep sleeping and they will meet again.
Eventually, Sora is brought to the real World that Never Was, where Xigbar explains how his journey had been manipulated all along, and Young Xehanort plunges Sora back into a deep sleep, showing him illusions of his own past. Sora then dreams of the many people connected to him, and when he chases the phantoms in his dream, he falls deeper into sleep. Eventually he confronts Xigbar and Xemnas, who reveals to him the true purpose of Organization XIII and admit that Nobodies can regrow their hearts over time. Although Sora defeats Xemnas, the fight weakens him to the point that he cannot escape from his deep sleep. Young Xehanort reappears before him, and he tells Sora how they had manipulated his journey. As Sora falls unconscious, Xehanort informs him that he will become the thirteenth member of the new Organization XIII.
Riku attempts to find Sora in The World That Never Was, eventually defeating Ansem and returning to the real world. He tracks Sora to the Castle That Never Was, where he finds Sora in Where Nothing Gathers. Young Xehanort stops Riku from reaching Sora, and he tells him about the true purpose of Organization XIII as vessels for Xehanort, as well as his role in gathering the members from across time, and his choosing Sora as the thirteenth member. The current members appear, but before Master Xehanort fully materializes, Mickey arrives and freezes the area with a Stopza spell. However, Master Xehanort possesses the body of his younger incarnation, allowing him to move and to summon a Keyblade. Young Xehanort faces Riku in battle, but Riku finally prevails. The time freeze wears off, and Master Xehanort fully revives on the highest throne, while Young Xehanort takes his place on another.
Master Xehanort then tries to convert Sora into the thirteenth vessel but is quickly stopped by Lea. After the failed attempt to acquire the final vessel, Young Xehanort along with all the other members return to their respective points in time.
Kingdom Hearts III
In order to conduct an experiment on the growth of hearts, Young Xehanort takes several of Andy's toys and traps them in an alternate version of their world called the Toy Box. He has a particular interest in pitting Buzz Lightyear and Woody against each other to learn about the relationship between hearts and bonds. The toys (accompanied by Sora, Donald, and Goofy) track Xehanort down to Galaxy Toys, where he admits the fraudulent nature of the world before summoning Heartless to take over several Gigas action figures and attack the group.
Once the toys find Rex in the game store, Young Xehanort has a Heartless possess Buzz and forces him to shoot at Woody. Xehanort dodges Sora's attacks while explaining his plan: to separate Woody, Buzz, and their friends from Andy, who was the source of their hearts through their close bond, so as to weaken them, the appearance of Sora and their distrust of him being the last straw until they could be taken over. He then traps Sora in Verum Rex: Beat of Lead. Young Xehanort takes Buzz through a Corridor of Darkness before being followed by Sora and Woody. They dispel the darkness of isolation surrounding Buzz through the light of friendship, teaching Xehanort that empty vessels can truly grow hearts. He disappears and leaves behind the King of Toys Heartless as a "parting gift."
Later when the seven guardians of light reach the Keyblade Graveyard they are confronted by Young Xehanort along with his elders self, Ansem, Xemnas, and Vanitas. The five of them summons a horde of Heartless and Nobodies for the guardians to fight as they disappear into darkness.
After Sora rescues Axel's heart from the Lich in San Fransokyo, Young Xehanort appears once again, admonishing Sora for blindly pursuing the Lich to save his friends, and warning that there will be a heavy price to pay for using the power of waking foolishly.
After Sora has rescued all of his friends they make their way to the crossroads at the Keyblade Graveyard where Young Xehanort appears along with the entire real Organization XIII. The final battles begin shortly after.
After defeating nine of the thirteen darknesses, Sora, Riku, and King Mickey arrive at the Tower of Endings at the end of the labyrinth, where they are confronted by Young Xehanort, Ansem, and Xemnas. Master Xehanort informs them that the nine clashes beforehand have produced nine keys, taking the form of the No Name and that the remaining four will be forged with the final four battles, waving off the heroes' hopeful protestations. Young Xehanort, Ansem, and Xemnas then attack the group. After a difficult battle, in which Master Xehanort frequently interferes via a storm of Keyblades, Young Xehanort is defeated and returns to his proper place in time, though not before again gleefully warning Sora that there is a high cost to overusing the power of waking and that his time is up.
After carrying out his duties in the future, Young Xehanort returned to his proper place in time at the Destiny Islands. While the rules of time travel prevented him from keeping his memories of his travels, they engraved themselves on his heart so that he retained a desire to eventually leave the islands. 
Young Xehanort has mildly tanned skin, golden eyes, and silver hair which both slightly spikes and falls past his shoulders in the back; it is styled similar to Terra-Xehanort's, albeit with four bangs framing his face, the back two thicker than the front two, and the two backward spikes on the sides are absent, the topmost one slicked back. He wears a black coat.
Young Xehanort's body model in Kingdom Hearts Birth by Sleep is a slightly-modified copy of Saïx's, though it is likely that this is simply due to data recycling rather than any deeper significance.
Young Xehanort acts about the same as his elderly self in the present, appearing calculating, reserved, and sophisticated, but also cold, ruthless and patronizing. The main difference between Master Xehanort and his younger self is that Young Xehanort acts with a youthful confidence and cockiness, bordering on arrogance with a hint of a sardonic sense of humor.
Young Xehanort has the ability to control time with enough precision to manipulate the flow of battle, either by slowing the opponent while unleashing a multitude of swift, powerful strikes, or reversing the flow of time to recover from damage. He is also an extremely skilled magic user and is able to cast powerful spells based on the elements of Fire and Blizzard, fire magical shots similar to those fired by Sora in Wisdom Form, or teleport himself around the battlefield.
Young Xehanort wields two weapons that bear a striking resemblance to Xemnas's Ethereal Blades. Unlike many of Xemnas's weapons, however, Xehanort's appear to have hilts. The blades are made of light blue energy, and the black handles are somewhat reminiscent of the Terminus and Ruination Ethereal Blades. The blades can be combined into a dual-blade or can extend to be lashed like a whip. Xehanort wields these weapons with a reverse grip. In his battle with Riku at Where Nothing Gathers, while possessed by his older incarnation, Young Xehanort wields a Keyblade that incorporates elements from both the No Name Keychain used by Terra, Ventus, and Aqua and the No Name Keyblade used by Luxu and Master Xehanort. He also wields this Keyblade during his battles against the guardians of light.
Notes and references