Inu to Kuzu  

Dog and Scum, 犬と屑

Haruma Sakuraba grew up with his best friend Shuuji Inukai, who always seemed superior in every way. Despite being an unremarkable salaryman, Haruma has spent his life feeling inadequate due to his inferiority complex. One day, he accidentally meets Reika Sumi, Shuuji's wife, whom he has secretly admired since high school. This encounter sparks feelings of envy, jealousy, and more, leading to the birth of a "secret" within him.