[@Prisk]
im down for that dynamic for marco and natalie. he'd definitely benefit a lot from her protection—especially post-diagnosis, when his confidence would have been a lot more fragile. marco would have been pretty intimidated by her at first, then surprised when she expressed genuine interest in his art. After the initial shock, though, he'd respect her—though he'd keep her distance for both their sakes: marco not wanting to draw any attention on himself from natalie's fellow mean girls, and not wanting to jeopardise natalie's reputation in case she was seen being too friendly with someone outside the popular circle.   

he'd have an instagram she could follow: scrolling through his posts would be like a timeline of his condition's progression, beginning with early selfies and portraits of his beaming, younger self, before gradually transitioning to simplistic videos of his piano playing, first showing nothing but his hands and then obscuring both them and his face. it'd still be up in the present day, though his posting's probably slowed down significantly. 

[@Gisk] marco definitely would have taken a shine to billy—he'd be instantly infatuated with anyone that took music as seriously as him. they'd have naturally crossed paths with their involvement in the school shows, and it wouldnt have taken long for marco to get attached (possibly beyond platonically? though marco would have kept that to himself) to billy from there. 
their friendship would have ended pretty quickly after marco's condition started to worsen, though: billy not keeping up with him would combine with marco intentionally shutting out a lot of people he knew before his diagnosis as his personality and outlook on things began to shift, so they'd likely have lost contact before graduation. 

present day, marco would probably be a little embarrassed meeting billy again, given how quickly he withdrew from their friendship. if he finally opened up, though, he would definitely go way too hard on the musical analysis of his band's discography. 

[hr]

as for the rest of marco's relations, today's been pretty busy and it's pretty late by my timezone. i'll try brainstorm some ideas for tomorrow, but in the meantime, anyone can feel free to hit me up if they've got something in mind for how their character felt about/interacted with him.
(i may also borrow [@Aeolian]'s amazing relationship sheet to kick things off, if they don't mind? lol)