Sayaka's backstory. A senpai at her old school asked her out, and Sayaka, having fallen in love with the older girl, accepted. They went out for a while, only for the older girl to break up with Sayaka, saying that they were starting to outgrow their Gay Romantic Phase. It would be bad enough if Sayaka and her ex stopped loving each other, but the worst part is that Sayaka's ex never thought of the relationship as permanent, meaning Sayaka had loved someone who never truly reciprocated her feelings. It's driven home even further in Special 3, when Sayaka's senpai apologizes to her for making her like girls, providing an uncomfortable reminder to Sayaka about how many people refuse to acknowledge same-sex relationships.
Touko's flashback at the start of Volume 4. The two sisters play rock-paper-scissors, and Mio, the loser, goes to take out the trash. Touko then falls asleep, and wakes up to realize that her sister isn't back yet- it's strongly implied to be the time Mio was killed, and suggested that Touko might blame herself for it. It gets even worse when you consider Sayaka's theory- that the reason why Touko dreams about such a distant memory is because she's thinking about it all the time.
In Chapter 21, Ichigaya informs Touko that while he was on the student council with Touko's sister, Mio was actually a slacker who made the others do all the work, including her homework. His intentions were good, since he still thought of Mio as a friend and was honestly impressed with Touko's work ethic and competence, but he ended up forcing Touko to question how well she knew her sister, and by extension, the goal that's been keeping her going for the past seven years.
In Chapter 22, after learning the truth about her sister, Touko reveals the depths of her misery when she says she hopes Yuu will never love her, because Touko could never love someone who loves what Touko hates- herself. Yuu, who all but admits to herself that she loves Touko, doesn't take it well at all.
In Chapter 27, Akari doesn't just have to deal with her crush rejecting her, but also learned that he'd already had a girlfriend and didn't even bother to be honest with her. The fact that this inspired righteous fury in Doujima of all people goes to show how much this hurt for her.
Chapter 34. Yuu, unable to keep her feelings in any longer, makes an Anguished Declaration of Love to Touko, whose initial response is to apologize. Yuu then runs off, while Touko tries to go after her, but is rooted in place by her fears and regrets.
All of Chapter 35. Yuu, apparently mistaking Touko's apology(for not taking Yuu's feelings into account) for a rejection of her Anguished Declaration of Love, begins to question herself and wonder why she isn't crying like Akari did, even though she's clearly heartbroken. Meanwhile, Touko, while acknowledging that Yuu "isn't the girl who couldn't fall in love with me anymore," and having taken Yuu's charm off her bag, realizes that she misses Yuu. The awkwardness both feel when together, barely able to say a word to each other, is especially painful.
In Chapter 38, Touko ultimately has to turn down Sayaka's confession. Touko does it as nicely as possible, while letting Sayaka know how much Touko values her as a friend, and Sayaka takes it fairly well, but all the same, it's fairly sad that Sayaka's honest and true love for Touko will not be reciprocated.
Regarding Sayaka Saeki
While Touko let Sayaka down as gently as possible, the end of the second volume of Sayaka's light novel spinoff reveals that Sayaka still hasn't completely gotten over it, and still feels pain from not having her love returned. The two are still friends, but there likely always will be some degree of awkwardness between them due to Sayaka's Unrequited Love for Touko. This is part of the reason why Sayaka gradually grows closer to her former romantic rival Yuu, of all people, than she is to Touko. The third volume has Sayaka's friendship with Touko mend somewhat, but Sayaka suspects that they'll continue to drift apart as time goes on.
Anthology Chapters
"Lies and True" chronicles Sayaka's Unrequited Love for Touko during the time when she's still struggling to come to terms with it.
Sayaka: (internal monologue) I know plenty of sides of Touko that Koito-san hasn't seen, too. That much is certain. But... I've never seen the side she shows her love. I can never know that side of her. I never had a chance... and I never will. (to Yuu) Honestly, there are times when I'd like to raise my voice. Yuu: Oh, r-really? Sayaka: Right now, for instance. (Beat)Not really. Yuu: C-Come on, don't scare me like that, please! Touko: Sayaka! Yuu! Sayaka: (internal monologue) I wasn't joking, but I wasn't serious, either. I swallow it all down, along with my voice. Because I still want to be able to smile when you look at me. So please give me a little more time. Until I can really smile from the bottom of my heart.
An In-Universe example is shown in "Touching!". Touko watches a play in which an old man meets his old flame's granddaughter, who looks just like her grandmother did when she was young, in a scenario reminiscent of Professor Layton and the Diabolical Box. The girl then proceeds to share her grandmother's diary, which details her feelings for the old man and why they could never meet. Touko claims her summary doesn't do the play justice, but what we can gather is already quite sad.
For fans of the series, the fact that the second volume of the anthology ends with Nio Nakatani declaring that with the manga and the spinoffs completed, Bloom Into You is finally over.
