Hello! I have quite a few student characters around Audrey's age so maybe I can help with plottage!
Millie Finnigan - I think Millie would be inherently distrustful of Audrey's motives because she's so different, so Slytherin and, at least in Millie's mind, not welcome in Potter's Army. I don't think Millie would want to be awful to Audrey but I don't think she'd be be disinclined to push Audrey's buttons in order to test her loyalty. Perhaps they could spend a morning in the courtyard duelling and spying on Dark Followers. Maybe that way they'll grow to get on. (:
Ellie Finnigan - Unlike his sister, Elliot wouldn't be that bothered by Audrey, I don't think, as long as she's not proving to be untrustworthy and "Slytherin". Haha. I don't think he'd really talk to her much but perhaps they can strike up a slightly reluctant friendship in class or during PA meetings? (:
Cerelia Avery - Cerelia is quite retiring, doesn't like confrontation much and is suffering confusion as to her feelings for Gisele Delacour and just .. everythingelseinherlife. Teenage girls. Pah. :3 Her father is trying to convince her to talk to more Slytherins though so perhaps Audrey, being in Potter's Army, would make Cerelia a little more up for a chat, y'know? I'm not really sure about plottage for these two. Hrm. Both share a conflict of expectations so perhaps they can bond over that!
Dae D'Eath - Daedalus is dying, technically, and he's quite unhappy about that. He also has to suffer the Malfoys and this strange relationship he shares with his cousin. In all of that, I think he'd find Audrey most interesting as she's definitely not what she should be - a cowed little Slytherin. He'd harass her a bit, I'm sure.
Isadora Kirke - These two may or may not know each other. There's a bit of backstory in Dora's app but in short, her family are disgraced Purebloods. She'd be hesitant to be around Audrey at first but I think they could definitely be friends! The more the better; anyone that's nice to Dora gains a friend! (: