Katie- Yes, they're friends, for sure. There might be some banter because of them both being on Scottish teams, though I think Montrose have a bigger rivalry with Portree. Random thought, if they go to events together/are seen together at them often, maybe the press wrongly assume they're dating. Could be a running joke between them?
Keira- it's safe to assume Emrys would be friendly with all the Hufflepuffs no matter their year. I'm not sure if he was a Prefect or not because I can't remember if Diggory was theirs (need to re-read) and he was the same year.
Morag- he might switch to her, he's thinking of designing his own line of Quid boots so if she can help hooking him up with contacts for that it'd be cool.
Fred- Rys enjoyed the Twins' humour at school so he is friendly, and was in the DA too. He'll be very shocked to see him if/when he does. But happy shocked.
Zach- definitely on friendly terms, especially as they played together on the 'Puff team.
Matthew- he works with Emrys' parents then! They're both in the DoIMC, his Mum is a rep for the French Ministry and that's where she met his Dad.
Mandy- depending on how long she's been at Mungo's she could have been one of the healers who worked with him after the Battle and during his recovery? He had extensive damage to his left ankle and wrist and needed physiotherapy and a lot of magical surgery to get his motor functions back.
Jack and Lex, they probably know each other in passing, but I'm willing to build on that in game!
Sera, Charlotte, Daphne and Penelope are likely to be random run ins. But they're always fun so I don't mind playing them out at all.