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In the AnimeEdit
Sayuri is Mai Kawasumi's only friend before she meets Yuichi, and thus her biggest part is in Mai's story. She is the daughter of a wealthy family, and always tries to smile and help others. She uses more formal Japanese honorifics for Yuichi when she addresses him, even when they become close friends. Sayuri cares very deeply about Mai, as it shows since they not only hang out together constantly, but Sayuri will get worried if Mai is in trouble; she also never forgets Mai's birthday. Initially, she is completely ignorant about the demons but even though some begin to target her near the end of Mai's story, she stubbornly stays by Mai's side. Sayuri, as with Ayu and Makoto, has a cute quirk; when she laughs, she voices a distinct ahaha. Sayuri always addresses herself in the third person due to an event in her past.
Despite Sayuri being a supporting character, she gets some back story to explain some of her current situation and how she came to become such good friends with Mai. Sayuri had been raised strictly by her father, and when her younger brother Kazuya was born, Sayuri decided to be strict with him as her father had done with her. Kazuya himself was slow to develop and did not talk even after entering kindergarten; he hardly ever laughed. Kazuya was always alone; he had a weak body so Sayuri would walk him to and from kindergarten. Kazuya eventually becomes sicker with time and is admitted to the hospital. Shortly before Kazuya's death, Sayuri gives him candy and attempts to play with him for the first time in her life. After he died, Sayuri began to address herself in the third person. Sayuri explained that she could only see herself from a third-person point of view from then on; she did not smile either until after she met Mai. Sayuri believes that Mai saved her and has been a much larger help in her life than what Sayuri has been able to do for Mai since they met. In the anime, a scar is visible on Sayuri's left wrist for only a second before she covers it up after she tells of her past. In the game, Sayuri had hurt her wrist when she was with Mai a while ago.
Sayuri's main involvement is during Mai's arc. Yuichi first meets her the day after he ran into Mai at the school at night for the first time. Sayuri asks Yuichi if he would like to have lunch with them, and Yuichi agrees. Upon discovering that Sayuri's food is very delicious, and because now Mai will have one more friend, Yuichi starts to eat lunch with the two third year girls every day for the rest of Mai's story. When Sayuri begins to notice that Yuichi and Mai are spending more time alone, Sayuri starts to suspect something is going on and tries to prove herself to Mai that she is able to help too. However, she ultimately gets seriously injured by the demons, but is able to fully recover. At the end of Mai's story, Sayuri graduates high school with Mai.
There are some alternate endings related to Sayuri. There is a hidden option to get a "best end" with Sayuri—to do this the player needs to listen to Sayuri's backstory mentioned above, and if the player does so Yuichi will later automatically choose to ask her out. The end of this storyline is a normal day except that Mai, Yuichi, and Sayuri are making a spectacle while eating lunch (Yuichi throwing food at Mai and Mai hitting him back) before the credits roll. One interpretation is that Sayuri is trying to move on from a less strict life.