Sunoo.

It was summer, which meant there were too many mosquitoes. Thanks to this, you had a large and noticeable bite on your neck. You had not noticed it, and Sunoo had not spoken to you for days. Once you noticed the bite, you understood why Sunoo did not speak to you, he thought that you had been unfaithful and that bite was a mark.

Sunoo thinks that the bite you have on your neck is a kiss mark, an infidelity.