Okay. So Kit used to be decently outgoing - not, like, super friendly, but enough that folks would know her. But now she's massively withdrawn and doesn't know how to interact. She's also a year behind so she's a 5th year, but she's sixteen. One thing most everyone probably knows about her by now: She was diagnosed with a terminal auto-immune disease. They may not know that the diagnosis was incorrect, or that she's actually not herself at all anymore. She doesn't like telling people about it, though, so that might take a bit of work. If you have anyone who's 18/a recent grad (or, really, her age and up) they might know her brothers and have heard she was released to go home and to school. Neither would have given much information, though.
Here's her app:
https://www.pottersarmy.net/t19030-avery-katherineIt's super long, but just maybe look at her Hogwarts history if nothing else. Or, y'know, peruse as you wish.
What she really needs is a couple folks she was close to before, or someone to try and get to know her. Because she's going to try not to have anyone get too involved, anyone who manages it should actually be rather impressed with themselves XD
And, I mean, really. Just look at her. She's gorgeous. Love her.