I've written before about Assassination Classroom, but I want to talk specifically about Nagisa, because he's fascinating.

For the first hundred-odd chapters, this is what we know about Nagisa. He's a kind, loving person, but he's weirdly casual about the prospect of violence--not like he wants it or enjoys it, but like he sees it as an everyday thing that's no longer surprising or scary. He's hyper-aware of when other people are too angry or worked up to reason with. He can mimic that same mood to others when he needs to, creating an impression of "I will kill you if you test me, so back off." He has feminine hair and clothing, but he strongly identifies as male, and his only explanation for how he presents himself is that it's not his choice. The other characters think there's something seriously wrong with him, but have no idea what.

Then we see his home life, and it turns out to be a survival tactic.