Kanda's Canon History and Backstory [FOR META KNOWLEDGE ONLY]
A product of the Order's "Second Exorcist" Project, Kanda is a "synthetic disciple," in the words of Bak Chan, engineered by the Order as a receptacle for the being (though not the consciousness) of an Exorcist killed in the war before him.
The goal of the Second Exorcists project was to create compatible Innocence-users by a kind of twisted recycling, and thereby swell (or at least sustain) the ranks of the Order and gain advantage in the war against the Millennium Earl. The Order had already tried and failed to simply implant Innocence into incompatible users and force them to sync with it (the tests apparently resulted in at least one child's death). In the Second Exorcist Project, they attempted to force synthetically generated bodies that were somehow linked (by literal brain tissue or, perhaps more metaphorically, by mind) to the Innocence-compatible dead to sync with Innocence themselves.
The process was an incredibly brutal one, and the repeated attempts and tests were only made possible by the regenerative abilities of the bodies subjected to it--the bodies of Kanda and another child roughly his age, Alma Karma. The deaths Kanda and Alma both suffered at the hands of the Order during his childhood must certainly have numbered in the dozens if not far more.
But worse still than this were the lies the Order's scientists and agents told to justify their actions, claiming that Kanda and Alma were saviors for humanity and hiding the fact that they were actually the product of a harvesting from the dead. Kanda had always been more angry, more hostile, belligerent, and more enraged of the two boys. He was never tractable, docile, or cheerfully accepting of their situation the way Alma was.
Whatever else their personalities say about them, Kanda's opinion of Alma's idiocy seems the expression of an inarticulate and utterly subjugated outrage at the situation they lived in. And yet the two boys became friends nonetheless--they had no one else but each other to rely on, no one who went through the daily tortures that they did.
It was during this time that Kanda began to have hallucinations-visions of a figure whose face and identity he could not make out. The woman he saw haunted his dreams where she appeared to him in a dying garden full of lotus blossoms, and the visions seeped into his wakeful mind as well. He didn’t know who this person was. What he knew was that the place where he saw her was one he knew, under a sky that he recognized without having ever seen. And soon enough, he knew that he was not supposed to know “that person’s” identity, and that the Order would rather destroy him than allow him to remember.
Kanda had begun to awaken to consciousness of what the Order had done to make him-his "core" memories, memories of his own death, had begun to return. The Order's solution was to euthanize him in order to keep their transgressions secret, in order to maintain the farce of their cleanliness and righteousness. They sought, in a very literal way, to kill off their mistakes and start clean by killing him.
They miscalculated though, and the euthanasia they had planned failed to destroy either Kanda’s body or the sense of self inside of it. It was in that moment, as he would not to die, that Kanda’s Innocence came to him. He awoke to carnage-to a mortally wounded exorcist (Noise Marie) who he unwittingly saved, and to the discovery that Alma too, in what had begun as a bid to save Kanda, had gained his Innocence as well as at least some of his own “core” of memories.
Mad with rage at his awakening to knowledge, Alma had killed all the laboratory staff, all the experimental subjects who had failed to awaken, and now intended to kill Kanda too. He wished to kill him so that he might die with him, in an ultimate retribution to spite the Order and to make them repent for what they had done. But Kanda was not so willing to die.
Forced to make the choice between killing his only friend and dying along side him, Kanda elected the former. He slew Alma, or so he thought.
Subsequently, Kanda became General Tiedoll's pupil and traveled around the world with him for a year before arriving at the Black Order Headquarters from his original "home" in the Asian Branch's Sixth Laboratory. Given what Bak says of "the tragedy nine years ago," he must have met Tiedoll shortly after Alma's destruction at his hands.
Of the many foes he has faced in his life as an Exorcist, Kanda has managed the considerable feat of defeating a Noah, Skinn Bolic, making him the only Exorcist to have accomplished such a task. This battle, which takes place on the Ark during its disintegration is won at great cost to Kanda: Mugen is shattered, his life force is deeply drained, and he doesn't escape the room before its collapse around his ears. However when Allen reconstructs the Ark before its final destruction, he also reconstructs the room from which Kanda had failed to escape, and the swordsman is thus saved.
Later, in a conflict in Jordan, Kanda is taken by the Noah to the North American Branch of the Order as part of their carefully orchestrated plan to awaken The 14th Noah inside of Allen.
It is then, as Allen is drawn with Kanda into his memories of the past, that Kanda comes to know that Alma didn’t die. Kept in a state of hibernation and used as a living tissue bank for the creation of hybrid “Third Exorcists,” Alma’s awakening becomes, in the end, the occasion for the answer to the question Kanda had been searching for all along: “that person” for whom he had been searching was the haunting and the trace of the previous life which the Order had transplanted into Alma.
In a monumental battle with Alma that follows his awakening, Kanda faces off against Allen, and blinded by fury, impales him on his sword, thus temporarily awakening the 14th Noah inside him. When Allen recovers, he uses the Ark to send Kanda and Alma (now a twisted being, nearing death, whose soul is being consumed by “dark matter”) away to a place where the Order cannot reach them. It is there, finally, that Kanda and Alma say their goodbyes, Kanda cradling Alma’s broken body in his arms, the vision that comes to him at last being of Alma and “that person” walking away from him hand-in-hand.