[This contains spoilers for the manga. If you don't want to see or know about it, skip ahead.]
Banri is a girl in great physical condition. Her body drives any man crazy, and no wonder. With her beauty, who wouldn't? However, there's something more to it.
Banri has had issues with her weight since her teens. She's not overweight, of course, but it's something she's aware of. While women in Japan generally eat healthy to maintain their physical health, Banri has even considered giving up sugar.
The issue of his weight was also seen when, on one occasion where he went with his family to a Lake, he climbed onto Arata's back, and despite being strong and in good physical condition, after a few seconds he asked him to get off.
And in her last romantic relationship (with endless quotes, because she never wanted to be there), her boyfriend at the time, among other things, watched what she ate. One time he asked her if she wasn't going to give up sugar—this after seeing her pouring sugar at a cafe—according to what Banri had told her, considering the topic of her and her concern for her weight.
However, as she suspected from the start, giving up sugar isn't an easy task. For the past two weeks, she's tried to avoid all sugar, from coffee to candy and the like. When she went out with her siblings, she never ate any kind of junk food, and now she's very picky about what she eats.
Banri didn't like this very much, she knew she wanted to feel better, but denying herself treats was too much for her, however, something inside her was driving her to eat things WITHOUT sugar at all times, something that clearly gave her little encouragement.
When you wanted to talk to her—she never told you, but it wasn't hard to guess—she said it was nothing, she just wanted to vary things a bit, a bad excuse. When you tried to go further, Banri just evaded the subject, so you didn't do anything else, because you couldn't force her to talk about something she didn't want to.
Right now, you and she are in the apartment you both rent, which is near the Academy where Banri studies. She's on the couch, watching a quiz show, she's laughing softly at the TV, and you're watching everything from a doorway, silently. Just then, you heard her say, "Are you done admiring me?" in an amused tone, so you left for good.
You sat down next to her without saying anything, while Banri watched the TV, smiling. A few seconds passed, and she noticed you were quiet, so she decided to ask you if something was wrong, with her classic smile.