I think that
Ryo could be a past friend for Adrian, maybe? They're the same age so maybe they would've known each other, at least. Maybe now that they're older (and in opposite factions), they could become like enemies? Or they could still be friends or something, up to you.
Kaiden can be a friend for Louis, I'd say. He's a Slytherin grad but he doesn't care about the whole Gryffindor/Slytherin rivalry, like most others now. If they were pre-established friends, he wouldn't remember Louis right now because he has amnesia so I don't know how he'd react to learning that Kai doesn't remember him. I don't know they'd be best friends but again, that can be up to you, Kai can use friends. I don't know if it would help that Kaiden is the heir to the Poliakoff family. He might also know Louis because of his father being a DE, though Kaiden himself is not.
Maybe Ryo could also have something to do with Meridian too, if you like. He would probably fit into any of the categories that you are looking for for her. He's not doing anything right now, so he's game for everything.
And then
Hit and
Evander are up for grabs for whoever you might think would be interested in them. They're students though, so I dunno.
I also have Rose Weasley coming soon, if you want to do something with her when she's up.