So let me start this off by saying that Susan never knows how to talk to other kids? He just hasn't been around enough of them, and most of the kids he sees are Very Immature compared to him you see (no they aren't susan...) and so that kind of colors his opinions of kids generally. Because you see he's a very mature just-turned 13-year old.

But Reira isn't like most of the kids he has met, by which I mean she isn't like... whiny or annoying or just childish. She's kind of weird, and he isn't sure what to think of her in all honesty. He definitely doesn't mind people who don't talk a lot (I mean look at his brother), but that still doesn't mean he knows what to think about them. But she seems mature enough and willing to do her job and that sort of thing, very independent, which he definitely likes and makes him think she's interesting.

...And now she's in trouble or something and although he's not clear on the details, he can't just let people be in trouble. So now she's kind of the damsel in distress in his mind, and who knows how that will end up in the future exactly. As long as he finds out that she was brainwashed into it or whatever though he won't hold the whole getting thrown in jail thing against her for long.

TL;DR he doesn't know her well enough to have much opinion but she's definitely interesting and he wants to help her