Hey, I'm Brooke, one half of the mod duo here at TNG. I bring you a bunch of characters, so let's see how they'll all play out here with Alex.

Jenny Jigger - 5th year Ravenclaw - I think they'd know each other, and she probably flirts with him (she's a bit of a flirt - she used to be chubby, but she lost a lot of weight in the past year so she's trying to make up for lost time with boys).
Emerson Toots - he's one of Alex's roommates, so I think they'd be friends. He's awkward around girls, but he's very driven to become the next minister of magic, so he's very politically minded. He's a sweetheart, though.
Ariel Windsor - 7th year Hufflepuff - I don't think they'd interact much?
Rose Weasley - 6th year Gryffindor - she'd probably like Alex if he was nice to her, which I'm sure he is. She's really friendly.
Diego Cartaya - 6th year Slytherin - he's a nice guy, but he's captain of the quidditch team so he's really competitive. Regardless, he probably gets along with Alex. You say that Alex is bisexual, though? I may take you up on this aspect of his personality at some point in the future for a plot with Diego (IM me and I'll tell you more about what I have in mind).
Neville Longbottom - beyond being his professor, I don't think there's much plotting here.