Time travel is a person or object's movement away from the present time and to a different point in time in the past or the future. It is used by multiple characters in the series, particularly various incarnations of Xehanort in order to fulfill their purposes of starting a second Keyblade War.
For someone to move through time, they must first abandon their body, becoming nothing more than a disembodied Heart in the process. Xehanort accomplished this task by turning himself into a Heartless, allowing this incarnation of the man to make the travel necessary to begin the recruitment of the Thirteen Seekers of Darkness.
The act of disembodying one's heart also allows the traveller to grant access to time travel to incarnations of themselves scattered through different points of time. However, so long as these incarnations are not disembodied, they are unable to travel through time of their own accord, requiring assistance from the disembodied traveller.
A unique method for time travelling was the doorway to Timeless River, which did not abide by the normal rules of time travel.
In order for one to travel to a certain point in time, there must be another version of them waiting at said destination. In addition, rewriting the events that are destined to happen is forbidden in the laws of time; what is set in stone cannot be changed. Upon return to one's original point in time, the traveler loses all memory of the experience gained traveling. In spite of this, the memories of these experiences are etched into the traveler's heart, which can potentially have an effect on their perspectives and decisions.
However, Sora was able to alter the events that were originally destined to happen, thanks to his interaction with Naminé in The Final World prior to travelling back in time, successfully altering time and saving the guardians.
The doorway to Timeless River does not follow these laws.
Known Time Travelers
With the ability to travel through time granted to him by Ansem, Young Xehanort uses his newfound power to visit dispersed versions and vessels of himself in many points in time. Able to gather ten of these individuals, he gathers them in the present day to join Xehanort's Thirteen Seekers of Darkness. The currently known incarnations who have been gathered are Ansem, Xemnas, Xigbar, and Saix, all of whom were defeated by Sora and Riku in the present day. In addition to this, when placed on Destiny Islands by Ansem during a time before it was destroyed by the Heartless, he is able to follow Sora and Riku into the Realm of Sleep. With Sora marked by the Recusant's Sigil and Young Xehanort present in the same time period as him, the youngest incarnation of Xehanort is able to keep tabs on him wherever he might be. This allows him to track Sora wherever he goes as part of his assigned task to turn him into the thirteenth vessel for the Thirteen Seekers of Darkness.
Terra-Xehanort is the body of Keyblade apprentice Terra possessed by the heart of Master Xehanort. During his apprenticeship under Ansem the Wise, he takes the opportunity provided by Ansem's trust and resources to further study the Darkness and find potential ways to gain further access to its powers. Upon banishing Ansem to the Realm of Nothingness, he takes his former master's name and falsely assumes his identity, then proceeding to separate his heart from his body, splitting himself into a Heartless and a Nobody: Ansem, Seeker of Darkness, and Xemnas, the leader of Organization XIII.
At some point before being split into Ansem and Xemnas, his heart was collected by Young Xehanort so he could be made one of the Thirteen Seekers of Darkness in the future.
Ansem's function as a Heartless, a disembodied heart corrupted by darkness, enabled him to travel through time, just as Xehanort planned when he cast off his heart to create the former incarnation. Using this ability, he travels to the past and arrives on his original self's former home world, Destiny Islands, to convene with a younger version of himself. With time travel powers granted to him and the rest of his incarnations by Ansem, Young Xehanort is able to travel through time without leaving his body behind. His task fulfilled, Ansem returns to his own time and places himself on Destiny Islands, where he places the Recusant's Sigil on a time travelling Sora.
Ansem later possesses Riku, reshaping Riku's body into a physical form designed in his image. Even after his initial defeat he remained within Riku for an extended period of time. At some point prior to his final defeat, his heart was collected by Young Xehanort so he could be made one of the Thirteen Seekers of Darkness.
Following her defeat in Kingdom Hearts, Maleficent took advantage of lacking a body, and with the information on time travel she had been given by Xehanort she traveled back to the events of Kingdom Hearts χ.
His heart was collected by Young Xehanort at the point in Chain of Memories were he believed himself to be the real Riku to be made one of the Thirteen Seekers of Darkness as Dark Riku.
Being a Nobody, Xemnas is not supposed to be able to travel in time, that's why he stayed in the present to found Organization XIII. Nevertheless, since Nobodies are able to gain replacement hearts, Young Xehanort was able to collect Xemnas' heart (from at some point prior to him merging with Kingdom Hearts) on his travels to have him join the Thirteen Seekers of Darkness.
Having lost a body following his battle with Ventus, the fractured heart of Vanitas became capable of time travel. He was collected by Young Xehanort at some point between the events of Birth By Sleep and Dream Drop Distance to be made one of the Thirteen Seekers of Darkness, though he was one of the last to be given a Replica body.
During the events of Kingdom Hearts III Young Xehanort traveled back to the time when Xion was first created and brought her heart forward to be made one of the Thirteen Seekers of Darkness.
Ending up in The Final World following his and the other guardians' deaths due to Terra-Xehanort and the massive Demon Tide, Sora and Kairi traveled back together to before the battle to change the events.
Ending up in The Final World following her and the other guardians' deaths due to Terra-Xehanort and the massive Demon Tide, Sora and Kairi traveled back together to before the battle to change the events.