Koji is a tall young man of a broad, muscular build with peach-colored skin. His head takes the form of a rock, which is unevenly shaped and pointed at the top of his head, and his jaw is square-shaped. From the back of his head, thick hair-like protrusions slope downwards. Contrary to his powerful and brutish appearance, Koji is timid and quiet. As such, he rarely speaks and prefers using sign language to communicate with his classmates, so much so that Kyoka Jiro assumed that he was mute.