He’d give ‘em a little scarf! Alternia’s nights can be really chilly and uncomfortable if you ain’t got some proper clothing. So he would make his matesprit a favour. Plus, it looks adorable on them!
For “possessiveness” he’d probably put his sign there, somewhere on that scarf. Maybe in both tips. Other trolls would just go “oh fuck that cute alien’s got a badass fucking quadmate and I don’t want to fuck with that guy if I try to get them for myself”. Because you too would be scared of a strong cobaltblood who can hack things, if you two weren’t actual lovers.