@sighoon
I actually quite disagree.
Manga starts from his perspective as he saves her. We know how he thinks but she's a mystery for quite a while. Characters don't start appearing until he meets them / interacts with them in some way. He's definitely the protag.
@Tamerlane
Why? She'll probably cheat on him with a girl later down the line. Why risk a closet lesbian? Dude's physically fit, well liked by his colleagues and is generally outgoing. I doubt he'll have problems finding a girl if he tried.
Also, to you guys screaming NTR.
It's not NTR if they're not already in a relationship. I doubt the brother would mind even a little if she got with some random dude/chick at school because there's no attachment between the two in that way.