Kira is a young boy of small stature. He has straight black hair that reaches his chin. He keeps his hair with one strand between his hair and the rest just hanging down. He has deep red eyes.
Kōsuke wears a cyan hoodie with a drawstring. There is a pentagram on the front with the word "Pentagon" over it. He wears a dark blue vest over his hoodie. As the series progress, it seems that the sleeves of his hoodie have been torn off. Beige cropped pants and black and white tennis shoes complete his outfit.
Kōsuke is sadistic, as evidenced by his cruelty towards his opponents and his massacre of frogs in his debut chapter. He tells Josh that he doesn't understand the value of the lives of small animals. His lack of empathy, as well as his delinquent childhood, may be because of living with his abusive father, whom he is cowed by.