Plot(from otp fluff generator):

Frostbear has scars on his  back & shoulders . One day Red-bear has his back exposed, and  you hug Him from behind, kissing down Red-bear's back. When Red-bear asks what You are doing, You reply "no one has ever kissed them better before, so I'm doing it now."

Scenario 1:

He looked back at you,  he was surprised 

Scenario 2:

" o-oh.. it's fine though.. they don't bother me that much.." his ear twitched lightly

Scenario 3: 

He tenses up, looking over his shoulder at you " oh..? it's fine though.. you don't half to.."