The protagonist of X, Kamui Shirou is a powerful esper whose destiny to decide whether the world should be destroyed so it may be reborn without humanity or save the world so humanity can continue to live in its current state.

In Kamui's childhood, he lived in Tokyo, across the street from the Togakushi Jinja.
Shortly after moving in, Kamui became fast friends with Fuuma and Kotori Monou. Kamui and Kotori showed affection for each other at a very early age, which culminated in Kamui asking Kotori whether he could become her "bride". He has sworn to Fuma to protect her and never make her cry, and in exchange Fuuma promised to protect him. In the TV series, this and several other childhood scenes from the manga were replaced by a scene in which young Kamui prevents Kotori from falling out of a tree at the risk of his own life.
After this incident, Fuuma, grateful to Kamui for saving his younger sister, pledged to Kamui that should he ever need help, Fuuma would come to his aid.

At the age of nine, Kamui left Tokyo with his mother Tooru and moved to Okinawa.
They lived in relative peace for six years until Tooru's death. As Tooru burned alive, preventing Kamui from saving her with her own power, she implored Kamui to return to Tokyo to seek his destiny.

Upon his return, Kamui appears cold, ruthless, and unsympathetic but this is an act to drive Kotori and Fuma away in an attempt to protect them. Kamui's goal is to retrieve the Shinken from the Togakushi Jinja and leave Tokyo as quickly as possible. Kamui comes into contact with Daisuke, Arashi, and Hinoto in quite abrasive manners, resulting in several pitched battles. After Sorata's and Yuzuriha's respective arrivals, Kamui's violent behavior is tempered by their friendly demeanors.

Kamui ultimately chooses his destiny as one of the Dragons of Heaven, forcing Fuma to join with the Dragons of Earth.

After Kotori's death and Fuuma's disastrous awakening as the other Kamui, he falls into a state of catatonia, directly paralleling Subaru's own experience during Tokyo Babylon.
After Subaru retrieved Kamui's consciousness from the depths of his soul, the two mutually identified with one another, having lived through heartbreakingly similar circumstances of the death of loved ones at the hands of a close friend or lover. Ironically enough, it is Kotori's death that brings Kamui's kind and compassionate nature back to the surface again.

Kamui has stated that his wish is to protect Fuuma and to return him to his former self at all costs—even if he has to injure Fuuma in order to accomplish this goal. In the TV series he achieves his goal, though it cost him his own life, whereas in the movie Kamui decapitates Fuuma, and remains the only character who stayed alive.
In the manga however, the Dragon of Earth Kamui, who can see other peoples' wishes, states that this is not Kamui's true wish, and Kamui won't be able to win against him unless he realizes and changes his true wish.
This is also given as the reason that Kamui is, as of yet, unable to generate a kekkai in the manga.

An alternate version of Kamui makes an appearance in CLAMP's crossover series, Tsubasa: Reservoir Chronicle, as one of the vampire twins Seishiro is after. In Tsubasa Reservoir Chronicle: Tokyo Revelation, he is voiced by Mamoru Miyano.

In the X Tarot set, he represents The Magician.

Voiced by: Tomokazu Seki (Japanese) in the feature film.

Voiced by: Kenichi Suzumura (Japanese) in the TV series.