I love thinkers.
Anyway, I read a little of both apps of yours, and here is what I think.
Seeing that Mira is older, hot and kinda like a badass, Damien will admire her a lot. He likes hanging out with older kids, and if you want, we can have a friendship where D admires her and she enjoys the attention. She'll always see him as a younger brother, kinda. He, however, has a soft spot for her. He knows that even if he likes her, she will never return his affections. He puts her on a pedestal, but they have a fairly good friendship. He confides in her, and struggles in loving her as an elder sister (kinda) and also with his little romantic affection for her.
As for Georgie, since they're in the same year, we can play them as really good friends, and that she was D's first friend at Hogwarts, and it just took off from there. Given their differences, they'll definitely disagree quite a bit. Sometimes he is annoyed with her, but mostly he is protective towards her. I'm not sure if you already have fixed plans for Georgie, but I think the 'want what he can't have thing' works with Mira for D. With Georgie, perhaps something romantic in return, potentially for the future?
Anyhows, D is actually not a player. He is too attached to be able to do that, especially when the girls are older. He has a lot of infatuations, but he never actually acts on them. Flirting, he'll do. More serious, or basically more physical stuffs, he does pull back from. He's not a have-them-and-leave-them person, unlike his father.