Kirichi

Kirichi is the het ship between Kirie Goshima and Shuichi Saito from the Uzumaki fandom.

Manga
In the manga, Kirie and Shuichi are explicitly stated to have been in a relationship since an unknown point before the start of the story. They are extremely close, and often confide in one another.

After his parents' suicides, Shuichi gradually wastes away, unable to sleep, refusing to eat, and deteriorating into an anxious mess. Kirie becomes his main caretaker, as well as the only person in their town who respects Shuichi and doesn't ridicule him.

As the two of them are constantly pitted against the Spiral directly due to its apparent interest in them specifically, Shuichi and Kirie frequently save each other's lives, but ultimately, at the end, once they are the last ones left after the Spiral destroyed and killed the entire town, they agree to die together. That said, they don't die; the Spiral freezes them in time together, and it is implied that they will be unfrozen once the Spiral awakens again, which, presumably, will happen anywhere from centuries to tens of thousands of years later.

Movie
In the movie, the characters are changed drastically; Kirie is an empoverished tenth-grader with no mother and a neglectful father, and is ignored by her peers at school. She relies on Shuichi, who is considerably older than her in this version, for emotional support. Though Shuichi assures her that he will protect her (both from the Spiral and from any other potential threat), he is shown to frequently belittle and terrorize her. In the end, Shuichi becomes contaminated by the Spiral and unsuccessfully attacks Kirie, and Kirie is left the last survivor of the Spiral.

Differences between Kirichi and Shuikiri
In the manga, Kirie and Shuichi are the same age (or at most a few months apart, as they are in the same grade), both in high school (before Shuichi drops out), and in the same socioeconomic class (middle or upper-middle). Their roles in their relationship are also switched in the movie, as in the manga, Kirie is caretaker to Shuichi and often plays the "dominant" role to him, as Shuichi in the manga is often too frail and too mentally ill to take care of himself. Kirie in the manga is older, more mature, more popular at school, and more self-confident then her movie counterpart, while Shuichi in the manga is younger, more fragile, less popular with the town, and less self-confident than his movie counterpart.

In the movie, however, Shuichi is considerably older than Kirie and is openly controlling and often dismissive of her. He has already graduated, while she is in the tenth grade, and he comes from a rich family, while she is empoverished and has only her neglectful father for family. In the movie, Shuichi is shown as being generally respected by the townspeople, while Kirie is often ignored. She relies on him for emotional support, though he rarely gives this (and if he does, it is more often than not deadpan and not genuine). Shuichi in the movie is fully neurotypical and self-sufficient, and lacks manga!Shuichi's psychic abilities. The fear of the Spiral is instead given to Kirie, who has a bad feeling about the town.

Fanon
As the ship is both canon and between the main characters, Kirichi is a very popular ship. Due to the lack of other potential love interests, there are no ship wars between Kirichi and any other ships, as the not-Shuichi characters interested in Kirie are stalkers (Mitsuru Yamaguchi, Wakabayashi, Hurricane #1), and the not-Kirie character interested in Shuichi is a sexual predator (Azami Kurotani)--however, complications arise due to the Split Base nature of the Uzumaki fandom(s). Kirichi shippers are very much at odds with reviewers of the manga (referred to as the Reviewing Fandom) and Shuikiri shippers, as Kirichi shippers tend to view Shuikiri as a toxic and creepy relationship, while Shuikiri shippers tend to dislike Kirichi because they find the characters "boring." The Reviewing Fandom are also infamous for misportraying manga!Kirie and manga!Shuichi.

Fandom
AO3

Wiki