I totally understand you. XD
Maybe Beth, she's just a year younger than him so they would probably saw each other when they went to Hogwarts, they're both weres and they both want to keep it secret as long as it's possible.
Or Athena, they both seems to be evil enough.
I'm not quite sure how Theodore and him would getting along.
Not Melissa, she knows his brother and Sebastien doesn't want to Chrisian knows he's back until he decides otherwise.
Sebastien would probably found Baldric annoying. Especially because of his personalty, and a habit of asking question. He's not interested in answering.
Hugo, ah, he would considered him a child, unless for some reason he prove him wrong then he would start respecting him as equal.
For Peter, they would probably seeing each other in Knockturn Alley, neither of them seems to mind of breaking the law.
Frank is, hm, too good?
James seems like he's messed up. That would make two of them. And both are addicts, too. Although Sebastien doesn't consider himself an addict, he thinks he just drinks and smoking cigarettes and that he can stop whenever he wants. But he can't.
Not sure about Elijah, he's too young for Sebastien to remember him from Hogwarts and I can't quite figure out his personality, so I don't know.
I went through all list, they're not sorted, except Beth, I placed her on first place because I read her app first and found it interesting. :3