Nanika Iino Mitsuketa! Re:

"I want to do anything for Koharu if that's what he desires."
Soon after transferring to a new school, Koharu is pushed down by the carefree and loyal-to-his-instincts senior, Oosugi. Before he knows it, the two of them are in a romantic relationship. Their overly affectionate, lovey-dovey behavior is well-known around school and continues to progress. However, things take a turn when Oosugi's friend, Shigihara, and Koharu's classmate, Yatsudo, get involved, sparking a love square battle! Meanwhile, Osugi's feelings for Koharu only grow stronger with each passing day...!?

(Source: Taiyoh Tosho, translated)

Note: Includes two extra chapters.