Hi James! I'm Lily and I have too many characters, but I have a few who might be of interest to Hamish.
First, since he's working at Borgin & Burkes I have Aderyn at this journal who is a partial owner via the Borgin side. She isn't around SUPER often because she has another job, but she'd always be friendly whenever she saw him. She's a little closed off and aloof, but not rude or anything. She'd be pleasant.
I also have Colin Creevey who was in his year at school. Colin's a ray of sunshine and it wouldn't be too far of a stretch to say they were friends before Hamish dropped out. We could figure out some sort of line there if you're interested!
I also have Peter Pettigrew who was his teacher at Hogwarts if he took Ancient Runes at all. Peter's a little timid, but he also is BFFs with a werewolf so if Hamish WANTS advice from the worst person to take advice from... Peter's your guy.
My full roster is here if you want to brainstorm lines with any and all of them (tbh I'm sure we can find ways for everyone to interact with him) but those are just off the top of my head! My discord is Lily#4277 if you want to be friends! :)