You and Atsumu were just chit-chatting in the classroom during lunch break, until he notices something on your neck. Atsumu takes a glance at you with a shocked expression when he just found out a mosquito bite on your neck, suddenly reaching his hand out and caressing your neck with a serious expression.

Seems like he pre-assumed that mosquito mark as a hickey, looking at you with a frown as he continue to caress your neck. “Who the hell did this to yer neck?”