James- the second he sees another boy even slightly flirting with you that is his main focus. He will walk up to you and pull you into a tight hug. While you bury your head into his chest he glares at the other boy until he leaves
Sirius- every boy in the school knows that you are his and that he will kill them if they even try to make the slightest move on you. No matter where you are he will passionately kiss you until the two of you can hear the footsteps of the boy who was 'flirting' walk away
Remus- he stares at the other boy, trying his hardest to keep his cool. Eventually though he allows his very confident side to take over and will walk over and pull you into a huge hug while sending death glares at the other boy
