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Tear Jerker: Sasamekikoto
Crosses over with CMoA in chapter 16, when Sumi snaps at Kazama. While it is (debatably) a bit cathartic to watch her stand up to Kazama, who she has continuously tried to hide her angst from, and tell her just how inadequate she has made Sumi feel, it still tugs at the heartstrings to see Sumi break down in tears and run away. It really drives home just how hurtful the whole issue has been for her.
Chapter 20 of Sasameki Koto, where we learn exactly why Kazama is so painfully Oblivious To (Sumika's) Love: the last time Kazama confessed seriously to someone, it was another best friend back in middle school, and said friend rejected her outright and called her disgusting. Kazama had to move and transfer to another school to get away afterwards. She's now so afraid of Sumika rejecting her in the same way that she fights against acknowledging her own feelings for Sumika and won't — perhaps can't — recognize Sumika's for her, to the point that she gives herself the Just Friends speech.
There's also the implication that she was bullied pretty badly after she came out, to the point where she had no friends left, which is a big part of the reason she transferred. Combine that with her dead parents and the fact that her older brother is usually working... Kazama's life was quite lonely before Sumika came along.
While it shouldn't be that surprising, given what he writes for a living, Kazama's older brother's unceasing support of his little sister's orientation always struck this trooper as heartwarming, especially in light of the bullying, the Parental Abandonment, and how he gave up a lot of opportunities to look after Kazama.
Oh God, chapter 46. Kazama tells Sumika that she has to move away to take care of her grandmother. This finally, finally elicits an Anguished Declaration of Love from Sumi, as she breaks down in tears, clutches Kazama to her chest, and begs her not to go. Kazama confesses that she loves her too . . . only to quietly apologize, insisting that she must. Heartbreaking doesn't even begin to describe it.
When Mayu tells Koi that she's in love with Sumika, Koi tells Mayu that they will always be friends, but latter when Koi is walking home she starts to cry, wondering why it isn't her.