Tsugi no Kuuhaku wo Ume yo.

"I never knew something could feel this good."

Kurokawa is cool and aloof, never showing his true feelings to anyone. His mysterious aura has made him quietly popular around campus. One day, Shirasaki, who shares a class with him, is asked by a group of girls to find out if Kurokawa has a girlfriend. But when Shirasaki approaches him, he notices a red lipstick mark on Kurokawa’s neck—a stark contrast to his "pure," "innocent," and "pristine" image…!

(Source: Libre, translated)

Note: Includes three extra chapters.