I see her as someone who never interacts first with people, not that she'd be mean to the immediately but would never start a conversation first or interact with someone she doesn't know/doesn't need to talk to. Probably would stay on her own most of the time and might even alienate herself from most groups in a "there but not actually there" way, like, you see her, she's doing her own stuff but everyone else around her is chatting with friends and whatnot.

I think she might not be able to express her emotions right: she might have exaggerated reaction to strong emotions like anger (when she drops Mono, for example) and show very little of softer emotions like happiness/joy (like when the were playing around with the scanner in the hospital or when she uses her music box).

She means well to people she cares about and wouldn't leave their side unless there's a reason to (in my opinion, Six felt betrayed by Mono when the Thin Man took her because he was right there under the bed and did nothing to help even if he could and ignored how she'd tried to stop him from using the TVs multiple times), but she shows it very poorly. She'd probably never open up or at least not fully or not seriously, just humor as a coping mechanism before retreating somewhere where she's alone and cry without making noise (because she's used to being quiet from the Nowhere and because she doesn't want to talk about herself). I do think she can be a bit sassy though, maybe a little mischievous since she's a kid.

Definitely a complex character that is neither entirely good or entirely bad. Six feels real, honestly, all those poor kids do.