Kamui's relationship with Kotori happens to be the reason why she got involved with Kamui's destiny. The two are not just childhood friends, but they are each others childhood sweet hearts. Kamui once swore to Fuuma that he would be Kotori's groom and he'd never let Kotori cry even at the slightest of time.
Kamui returns Kotori's love, perhaps more than even she ever understands. Initially, whenever his softer side would emerge, it was because of Kotori. It is for her sake, as well as Fuuma's, that Kamui attempts to battle his loneliness and remain distant from the only living people he loves and trusts. It is also for her sake, for the sake of protecting the Earth she loves, that he decides to become a Dragon of Heaven. Kotori and Fuuma are the center of Kamui's world--a world which is unfortunately tainted with tragedy. He cannot help but be drawn to them, even though the horror of losing them, of ruining their lives, seeps through his conscience like poison. Hinoto even says that Kotori is the person he loves most in the world, but she is also the person he cannot help but destroy. Even after her death, she continues to affect many of his decisions because it is partially for her sake that he wants to regain Fuuma. He may not have been able keep her safe like he promised, but he can potentially save Fuuma.
Kotori and Kamui become friends quickly and accept each others differences. Kotori with a weak heart and Kamui without friends, because he doesn't know who his father is. For years they grew up together. In the anime; one time, they are climbing up an old tree to return a fallen little bird from its nest; they fall but Kamui managed to hold onto the tree and to Kotori. Kamui told Kotori that he would never let go. Until afternoon Kamui didn't let go off Kotori. In the manga; there are two scenes that show their relationship. First is when Kotori climbs up a tree and she falls but Kamui catches her in the ground. Second is when Kotori's favorite hat was blown by the wind and got stuck on the top of a tree, because Kotori is afraid that Kamui will just get wounded. But then, the next day she just find her favorite hat on there class' roof near her room's window. She thought that it would be Kamui who'd try to get it and it is the truth. Kotori finds out Kamui wounded.
Kamui returns as a changed young man, a kind-hearted but troubled esper. Kotori finds out Kamui's return through her dream and Fuuma meets Kamui on the school grounds. But it was also Kotori that first confronted him upon his arrival. Kotori approaches him to let him know but Kamui asks for Kotori to leave. Kamui is still kind and changes his attitude to a scared one that he uses everytime he carries the unconscious Kotori to the clinic. Kamui also keeps Kotori from falling unconcious on the hallway of the hospital and embraces her tightly until she wakes up. Kamui also walks her home and make sure that she is just alright. For Kamui, his main objective is to make sure that the people that he treasures most, Kotori and Fuuma are safe and not in danger. Just by knowing that they are fine, brings him relief even he is severe wounded just because of the two of them. Since then, Kamui's attitude differs from Kotori and Fuuma. He only wishes that Kotori and Fuuma's happiness be maintaned. When Kotori asked Kamui about his wish since Kamui already knows Kotori's wish to be an indigo dyesmate, Kamui just answered "I don't have any wish of my own." Until his battle with Seishirou Sakurazuka involves Kotori; when Kotori is about to fall, he still holds Kotori's hand to avoid her fall and Seishirou stated that, "You are too kind but if you continue to protect that girl from falling, your defend will break and cause your own death." Still, Kamui would sacrifice his own life just to protect Kotori and Fuuma.
Certainly her frailty and her inability to actively shape the future constitutes part of her tragedy, but really, Kotori's main misfortune lies in falling between two young men who are destined to fight each other for the sake of the world. In her heart she knows that the fate of the world rests on her body and she wishes to shatter before destroying the world in which Kamui and Fuuma live. Even in death, she still held on to the hope that everything could work out for the best. Although Fuuma murdered her in an extremely violent manner, she still loved both him and Kamui. Her final words, words that resound throughout the story in opposition to the words of many others, were, "The future is not yet decided." Perhaps these final words will determine how this story ends.