Kamui Uehara is a metaphysical conception that is key to several games that exist in Kill the Past. Many characters in the series, such as Format Kamui, Kamui Fujiwara and Uehara are referred to by the name Kamui Uehara or Kamui at various points in time. Although it is suggested that Kamui Uehara is one person's personality moving between multiple people, this is unlikely to be strictly true, as rather than individuals becoming "possessed" by Kamui, it is more like that they begin to take on attributes of Kamui's supposed personality on top of their own.
The original incarnation of Kamui Uehara, known as the Format Kamui, was born on Lospass Island. At the age of two, his left eye was replaced with a silver eye in accordance with ancient Sundance Tribe rituals. Eventually, when he was in Kanto, he was killed in the event known as the Silver Case. In an attempt to replicate Format Kamui's personality as well as his silver eye, the Shelter Kids Policy was put into place. Because those who have acquired a silver eye can have their personality transmitted into others who are primed to receive it, Ward 24 was inadvertently turned into a breeding ground for the concept known as Kamui Uehara. A counterpart to Kamui was also developed in the form of Ayame Shimohira, which consists of the same Kamui personality being transmitted to a female host.
Under these circumstances, known as the Kamui System, the government could repeatedly activate Kamui within anyone who had grown up as a Shelter Kid, which they could use for assassinations (the political factions) or to make a statement to society (Nezu, who used Kamui as the equivalent of an 'in-game event'). Because the fact that the Format Kamui was killed was never made public, the people believed these Kamuis to all be the same one. Publicly, Kamui was sent to the psych ward of IMM Hospital in 1984. The circumstances regarding the emergence of Kamui's personality in this system are not detailed, but they involved the crime ring TRUMP, Kamui's "bodyguards", going to the Triangle Towers in the Shelter.
After Nezu's death at the climax of The Silver Case, Kamui is freed from this system. Now, Kamui and Ayame are concepts that manifest less rigidly. It can no longer be said that Kamui inhabits a single person, and is instead more of an omnipresence.
The reality of what Kamui Uehara truly is and represents is not clarified, but it is highly suggested to be something greater than a recurring personality. The word "Kamui" is sometimes used metaphorically, especially in Flower, Sun, and Rain and The 25th Ward in which characters other than Shelter Kids are called "kamuis" by other characters. At one point, Sundance Ritz recounts the events of the Silver Case as if they had happened to the Sundance Tribe as a whole instead of just to Kamui, with the Lospass hyenas representing Kamui's single silver eye. Kamui is frequently associated with God, likely due to the name's origin to refer to gods in Ainu mythology.
In the first half of The 25th Ward, Kamui is heavily associated with oncoming chaos. His existence is treated as a threat to Ward 25's ideal of peaceful orderly "life". In the series as a whole, the people look to Kamui as representing unpredictable change, which stands in opposition to the controlled experiments that hide behind the facades of the Wards and other conspiracies. He's also often associated with criminal power; the Format Kamui was known as the King of Crime.
In #05 electride, Kamui is referred to by Mokutaro Shiroyabu, who is attempting to eliminate him, as living inside a point. This conception of points seems to refer to a kind of invisible existence that allows messages to be implanted to those ready to receive them; whether becoming Kamui in the case of the Shelter Kids, or reading the messages and potentially utilizing his power in the case of observers like Kosuke Kurumizawa. Kamui is also described as the "main character of all of this", a sort of omnipresence that exists throughout the game's reality. It is unclear if this metafictional conception of Kamui was brought into Kanto by the Shelter Kids Policy or if the Format Kamui was just a manifestation of it as well, but it suggests that Kamui Uehara may represent the player in some form. For example, it is said that Kurumizawa "exists within Kamui's power", and he hides within the player's power by acting as the go-between between the player and the game in the form of the game's cursor.