[That just gets her a lengthy, unimpressed state. "Lightening the mood" isn't really something that tends to work with Shun bar rare cases with people he's close to.]

Or I'm saying it because you're making assumptions about things you know nothing about. [At the very least, he doesn't sound hostile, just blunt. It could be a lot worse, considering.] Besides, I didn't say I didn't care about them, just that I'm not the kind of person to be "nice".

[He's not exactly versed in the specifics of how people use "tsundere", but he figures it's a bit more extreme than his behaviour.]