In Miyu’s world, the Church was in the age of [Frozen Silence] thus, as religion and faith was gone, the Church lost its meaning and Kotomine became a shell of his former self that is simply observing to witness the end of the world. He notably found a meaning to his life by owning a Ramen shop that only makes and serves mapo tofu.
When he acts as a Ramen Shop Owner, Kirei wears a blue bandana and a blue apron after removing his habitual cassock and golden cross.
While Kotomine had dropped some his former duties, where he does not directly involve himself in any magical related issues, he still kept his duties as a priest to help people, to some extent, as he saved the dying Shirou Emiya and later informed the youth about the history of the Ainsworth family, their goal, and what they plan to do with Miyu.
Notably, unlike his counterpart, Kotomine seems to have a very acquisitive nature where he charged Shirou a heavy sum for two black keys; this acutely rapacious trait seems to be a recurring gag such as how he was going to make Tanaka and Illya literally pay with their bodies, for a bill of 3,200 yen, by taking their organs, though the two were saved by Gilgamesh in end, along with how Kotomine charged Gilgamesh soy ramen and mapo tofu, for five people, at a price of 55,000 yen, with tax.
Fate/kaleid liner PRISMA☆ILLYA Edit
Durant the 5th Ainsworth Holy Grail War, Kirei saved Shirou after he was attacked by Angelica Ainsworth. Waking up, Shirou first mistakes him for his father to which he responds by apologizing because he isn't the said person. He then says that he is a priest that helps drowning people. After changing, he welcomes Shirou to the Fuyuki Church, presenting himself as a priest that is responsible for witnessing the end of the world. He mentions Miyu Sakatsuki and Kiritsugu in order to keep Shirou in the Church. He proceeds the conversation by explaining to him the identity and goals of the Ainsworth family and the meaning behind the Holy Grail War, claiming that there has already been 4 rituals that ended in failure and revealing that had the worst outcome possible, destroying completely Zachary Ainsworth the previous head of the Ainsworth family, the Matou family and the Sakatsuki family. After the discussion, Kirei tells Shirou where was Miyu at the time they were talking, giving his details about the displacement field.
When Shirou won the 5th Holy Grail War after defeating the Doll Zachary Ainsworth who was using the Saber Class Card, Kirei comes to congratulate him, commenting that he became quite like Kiritsugu. He reveals that the opponent that he defeated was Julian Ainsworth's father, praying for his "pitiful" soul to rest in peace and telling to Shirou that since he already have chosen a path, he doesn't need to know the identity of someone that he cut down. He then tells him to go see his sister at the Mount Enzou, saying that this place is where is fate awaits.
After Illya's arrival in this world, he founds her with Tanaka in front of his Ramen shop. After their talk about the crater at the center of the city, he asks Illya to pay for the mapo tofu that he served them even if they have to pay with their owns bodies literally before being interrupted by Gilgamesh who wanted to know of the shop was open. Later on in the events of the series, Gilgamesh orders soy ramen with no mapo from his restaurant and he brings it (with mapo tofu) to their hideout. When he was paying the sum with a gold coin, Gilgamesh tells him that he've been to the Ainsworths to which Kirei responds with a serious tone to just leave the bowls in front of the school gate. During the second attack of Illya's group on the Ainsworth household, Kirei is seen on a motorcycle in the city after noticing the Giant Black Cube. He questions himself if they were able to push the Ainsworth that far into a corner or if Julian just does not feel the need of hiding the cube anymore, continuing by claiming that it's thoughtless from his part because he'll be found out. He finishes by leaving the scene and saying that regardless of the situation, it's not something a simple ramen chef should get involved in.
Kirei was never seen fighting yet in the series, making unclear if he had the same combat level as his counterparts. He still is in the possession of Black Keys and has a great knowledge about the Holy Grail War history and system, the Ainsworths' way of using Magecraft and the black cube.