The fictional characters of the X manga series were created by manga group known as Clamp, composed of Satsuki Igarashi, Nanase Ohkawa, Mick Nekoi, and Mokona Apapa. X takes place in the year 1999 when the end of the world is fast approaching as superhuman individuals gather and take sides in the city of Tokyo, site for the battle of Armageddon. Most of the series' characters with Kamui Shiro's exception originated from Clamp's dōjinshi they created before creating X while characters like Subaru Sumeragi appear as returning characters from the manga Tokyo Babylon meant to support the lead due to his parallel and tragic past. Ohkawa wrote the script while the other three members made the art.

The series follows Kamui, a young esper who goes back to Tokyo to fulfill his role in the Armageddon. He can join two groups for different objectives: either Dragons of Heaven and protect mankind from being extinguished, or the Dragons of Earth to protect the planet at the cost of ending the society. Kamui joins the former due to his only wish being protecting his childhood friends Fūma Monou and Kotori Monou. However, this causes Fūma to develop an alternate personality as Kamui's rival, filling his empty role in the Dragons of Earth and becoming his nemesis. The war between the two factions who involve seven members who each side begins. Due to the manga not being finished by Clamp, a film and an anime television series have provided the manga an alternate ending where the outcome and fate of the cast is created.

Critical reception to the series' cast has been positive due to their role in the armageddon, Kamui's early characterization also earned positive responses for his brooding nature until he becomes a more heroic warrior and his past is explained. As the film provided little focus on the entire cast with the main ones' exception, the television series was praised more for giving each member from the two Dragons their own screentime to explore their personalities like the manga did. The Japanese voice actors for the cast has been met with positive response though the English dub received mixed reactions for not fitting their roles.

After finishing Clamp School Detectives, the manga artists group Clamp decided to write a story in which readers see the development of two groups, the Dragons of Heaven and the Dragons of Earth led by Kamui Shiro and Fūma Monou, respectively. Ohkawa specifically chose the idea of seven characters from two groups because she was influenced by religious groups but wanted to avoid Christian references. Instead, she created the concept of the Dragons of Heaven and Earth and avoided less characters due to similarities with the tokusatsu genre. However, rather than starting the manga with the war between the Dragons, she instead envisioned the "high school manga" to introduce the cast in a more fashion mode. Kamui was originally written as a high-school student from Kotori Monou's point of view to appeal to the audience of females. However, controversial response from readers led to Clamp changing their characterizations. Ohkawa also aimed to show characters from their previous works in the X resulting in multiple crossovers. As Kamui and Kotori's characterization were changed due the negative response, Clamp wrote the idea of Kotori's post traumatic disorder and her eventual death which resulted in sad responses from readers. Though he had few appearances in the beginning, Ohkawa envisioned Fuma's transformation into the series' villain which the other artists from the group enjoyed and thus looked forward to draw.

Several of the series' characters were created using the Osamu Tezuka's Star System technique were old designs incorporated in new characters with Kamui's exception which proved challenging due to his role. He was made to stand apart from other characters, and Ohkawa called his hairstyle and school uniform average. Clamp's lead artist Mokona believes this was influenced by the heroic character-type upon which he was based. During serialization of the series, Clamp found issues with the amount of gore they aimed to portray especially Kotori's death which is foreshadowed in dream scenes. This was mostly affected by the themes of violence and video games present in the 1990s but the writers feared that toning down the violence would negatively affect the manga. Another death scene that left Clamp facing issues was when Fuma decapitates Saiki which resulted in more negative response from the readers. Ohkawa claimed that they were meant to be cruel with the narrative which did not fit in the magazine's demography. The fight sequences were inspired by the manga Dragon Ball most specifically by how the author Akira Toriyama used white backgrounds.

Kamui is prophesied to return to Tokyo as one who will determine humanity's fate. The construction of Kamui as a messiah is reinforced by his miraculous birth and given name; "Kamui", like "Christ", alludes to the character's nature. A common theme involves the series' fate, Subaru Sumeragi expresses no interest in the future of the Earth, but still he and his counterpart are drawn to Tokyo on the Promised Day. As a result, even with the fatalist atmosphere that persists in the series. His rivalry with Seishirō Sakurazuka parallels Kamui's rivalry with Fūma Monou. Clamp referred to Kamui and Subaru as siblings; Kamui is supposed to learn from Subaru's final fight with Seishirō and avoid his final fight against Fuma having the same tragic conclusion. Ohkawa also applied to Kamui and Fuma ideas she had during middle school such as the fact that both possess a dual nature as she states people can be considered good or evil.

When drawing characters, Mokona felt Fuma was the hardest one to draw as they often had to make him look like other characters like Kusanagi due to members from the cast seeing similarities between the others who they cherished. As a result, they avoided the idea of Aoki meeting Fuma since they would be forced to make Fuma androgynous due to the potential need of drawing Aoki's wife. Igarashi found drawing the Dragons of Heaven and Earth at the same time proved difficult because of their multiple unique clothes which left her wishing they instead wore the same outfits.

For X animated adaptations, the characters experienced changes from their original versions. For the 1996 film, Ohkawa helped director Rintaro in writing the script. The character Shogo Asagi was created exclusively to the movie. The characters were designed by Nobuteri Yuki. Director Yoshiaki Kawajiri aimed to portray Kamui and Kotori as stronger than their manga counterparts. However, he still wanted to highlight their psychological weaknesses across later episodes. Kenichi Suzumura had a poor understanding of Kamui during early recording of the television series as the pilot original video animation gave Kamui small screentime and dialogue. Fuma's actor, Junichi Suwabe, faced difficulties in voicing him due to his different alterego that makes him look like two characters. Koshinori Kanemori adapted Clamp's character designs and served as art director along with Yuji Ikeda.