Woah, a reference to Full Metal Panic? That's surprising. Nikaido and Yuya have good taste, that series is great!
Seems we're getting a glimpse as to why Hitotsuba suddenly announcing to the world Yuya was her boyfriend might be a bit too much for the school. Yuya was probably planning on taking things slow at school with Hitotsuba. But then this new person just made things move faster than he wanted.
It seems he's wising up to Hitotsuba's behavior at home. He actually made her back down with his words! But he's careful not to play too much into her games, or else he might fall too.
Shinji and Nikaido look like they're really good friends of Yuya. I almost thought Nikaido was Shinji's girlfriend with how it was framed, but I'm 75% sure that Shinji's girlfriend is the girl who appears at the end.