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.