A little bit of Shokugeki no Soma, a little bit of Menhera, a little bit of physical grooming... uh.
But that aside. I'm getting the general idea that this is basically a case of someone who unintentionally is anorexic and hopefully getting back to a normal healthy BMI, with the added bonus of very lewd erotic cosplay shots, childhood friend waking you up straddling you and showing off her belly shots like the one we just got, and foodgasm shots, and many more to feed the Suggestive tag.
Seriously though, eat well and stay healthy everyone.
I don't get it? The mc only now meets her, let alone recognizes her as his childhood friend. But she's his next door neighbor? What kind of logic is this?
Also there is no logic. We are here for the thinly veiled fetish that the author has and we're just here for the Suggestive tag. Everything is just setup for that, if it makes any logical sense by incident, it is purely by incident unless it becomes relevant to the story later on.
But if you want logic. We can reverse engineer our way through and thought exercise how it could make sense. Maybe they just kinda stopped interacting after elementary school, they were just kids after all. They both got hobbies that lean far more towards introverts and so they end up cooped in without interacting as much as they did in a shared space like the school playground back in elementary school. Even if they're neighbors, if they both default to being introverts that just stays at home playing games, the odds of meeting outside by accident reduces drastically. And if most of her going out is for her cosplay hobby, it reduces the odd further of being recognized.
So how plausible this is increases quite a bit. Now the better question is, how did she barge into his home and straddle him to show off herself? Did she get assistance from the parents who thinks he finally got a girlfriend?