@nasif just a heads up this does have the seinen tag, it's also marked with drama. Although keep in mind that seinen is a demographic (men in their late teens to 30s) and not a genre. So when two manga have that tag it doesn't mean that they'll be the same, just that they're aimed at an older audience (for example Berserk and One-punch man are both seinen but have wildly different tones.)
With the backstory bit, keep in mind we're only on the fourth chapter. It's most likely too soon for us to learn her backstory and we probably will only get hits at it for quite a while.
On real world people acting like this, keep in mind that:
1. Both the girl in this manga and the one in Its not Meguro sans first time seem to have special circumstances. They aren't acting in these ways because they are dumb, something happened to them to make them this way.
And
2. While the degree depends on the manga, most manga characters aren't going to act like real people. If they did a lot of stories just wouldn't happen. Plus keep in mind that most characters in manga are exaggerations of the kinds of people you find in real life.
So no, people usually don't act like they do in these manga