The true question is, did the Otaku-kun give her the black eye, or did she get a black eye from her male "friends", and that's why Otaku-kun wants to know who she was with and why he wanted to bring her home?
I can easily see the plot twist being that the Otaku is creepy because he wants to help the FMC, but it would also work if he were just being a creepy dude.
Why does it sound like so many of y'all have not read the literal work that came before?
The otaku,in his own weird way without a doubt,is 'bout to save her from those abusive...friends?,family?,some other third party?...and possibly do it with extreme injury to himself.