Sendai is interested in Miyagi, even to the point of pushing her way back into Miyagi's life after she decides to cut off their relationship when pushing a bit too far over the line. If anything, the more that we see, Miyagi is the one constantly running away and Sendai is the one doggedly chasing her down until she gives up, which is exactly how their strange little arrangement came to be. They're both pretty shitty people in their own ways, but so far from what I've read, it feels like they're just two lonely people who had the (mis?) fortune to meet a kindred soul to fill up the void they have outside of the classroom. I've enjoyed seeing their dynamic changed from the initial impression I got.

That being said, I swore this author had a bit of a foot fetish, in the beginning, but that died down in about 15 or so chapters. Who knows if it'll come back after chapter 35. (update: it did, but it's a lot less frequent than the first time around...)

At this point where I'm at, they've finally had their first kiss after a lot of anxiety, but haven't acknowledged a relationship past Miyagi's stubborness against calling each other friends (Sendai thinks they are, at least. Or at least she did before Miyagi said they weren't a few chapters prior).

Such an example includes why Miyagi can't trust Sendai:

"Because up to this point, every rule that we've made was broken by Sendai-san."

It's honestly just so heartbreaking to see Miyagi pull back so many times, despite Sendai going leaps and bounds to express her love, because of her fear of abandonment and the subtle trauma she holds from her mother and father's neglect. And Sendai, who has given her so many things--her earrings, a person to share meals with, a full round cake without leftovers--makes her keenly aware of the stark future she will have when Sendai no longer stays by her side.

like the amount of attention and care she puts into their relationship despite thinking it's one-sided and without Miyagi even mentioning what's wrong has made me cry more than once. Like when she chose Miyagi's earrings:

"But, if, someday, she were to love someone else other than me.

My heart wouldn't widen enough to wish for her happiness. That's why, when I can't wish for her happiness, I thought that this piercing could wish for her happiness in my place."

And all of this is because---

"Miyagi, like a stray cat, slipped into my high school life that was riddled with cracks, slowly warmed up to me, and gave me all that I am. If she wasn't here, I think I would have continued living in that monochrome world, a fake smile forever plastered on my face."

The backwards step from their relationship as high schoolers to roommates is quite simply explained. Up to this point, practically all of their interactions up to this point has been in the context of the 5, 000 yen bill, which muddles up their sense of boundaries and where they're allowed to cross. Living with someone isn't the same as spending hours after-school together--and for Miyagi, who very much enjoys her space, it's easily understandable that the first few weeks will be rocky as they adapt to each other's living styles and re-contextualize their relationship as roommates.

That being said, this relationship progression is very much on Miyagi's terms, and given her reluctance to be vulnerable to other people, it takes a very, very long time for them to move up steps in their relationship--and this speaks volumes of Sendai's patience and kindness as she waits for Miyagi to open to her.

The girls have moved in together and are attending college... but at this point they're "roommates" One of them has realized they're in love with the other but there's still 129 chapters left. Think about it. they've been together since the second year of high school. That's almost 3 years (2 years of high school, plus their time in college) at this point in the story. When high school ended, their "contract" was over and Sendai convinced Miyagi that she wants her to move in with her, to go to college together from the same apartment. This is a sweet moment that felt like their relationship was moving... BUT NOOOOOOOO, they're just roommates now. The romance/physical relationship dead-ends because instead of being afraid of breaking the rules of the contract, they're afraid of breaking the role of roommate? WTF?! They've done so much together, so why write it like this?

They also graduate from high school about 1/3 of the way into the novel, which is a win for any romance novel.

In the beginning we are lead to believe that Shiori is the main character. Certainly, with a title like "buying my classmate" you would expect a fetish novel with a major emphasis on the sadistic dominant main character. But after a few chapters, the story turns you on your heels and reveals that she's a deuteragonist and nowhere as threatening as she tries to make you believe. Hazuki is actually the one with agency in this story. Because of just how messed up Shiori is from parental neglect and her fear of intimacy, she gradually loses her ability to control the progress of her relationship, essentially surrendering the plot to her partner. By trying to drift away from Hazuki, she shies away from all her responsibilities, including her role in the story and connection with the reader, in a way that parallels Hazuki's feelings.