Oni Gal-kun no Hara no Naka

Ritsu Kamishiro, a diligent high school student, devotes his days to the restoration of an abandoned kendo dojo. However, he harbors a deep-seated insecurity about his height. One fateful day, a peculiarly attired man with flowing locks materializes before Ritsu, proclaiming himself as a demon. Startled, Ritsu learns that this enigmatic figure intends to reclaim a hammer from him, as he is believed to be a descendant of the legendary One Inch Man. The encounter leaves Ritsu feeling an intense heat and a tightening sensation in his stomach... He finds himself captivated by the man's irresistible allure.

(Source: Julian Publishing, translated)

Note: Includes an extra chapter.