Kotarou is a young boy genius and one of the main characters. He is often seen wearing a black catsuit that he built himself, modeled after Kuro but equipped with bulletproof and insulative features capable of withstanding high voltage. With an IQ of 200, he is highly skilled in technology and robotics, and his intellect rivals that of Dr. Go. Before becoming involved with Kuro and the others, Kotarou was an outcast who used his intelligence and a passion for retro gaming to conceal his loneliness. This background drives his initial obsession with Kuro, as he constantly tries to prove that he can be just as powerful. His naivety and lack of common sense, however, often lead him into trouble despite his earnest desire to make friends.

Kotarou begins as an idolizing fan of Kuro but later becomes inspired by Dr. Go's cyborg science after helping to rescue a character named Dunk, eventually becoming the scientist's apprentice. In the story, he serves as the team's inventor and technician, assisting Dr. Go and Mi with their inventions and building his own creations. He also acts as Kuro's unofficial mechanic, providing technical support and repairs. Though his early motivation is rooted in a need to prove himself and overcome his loneliness, he gradually shifts toward a genuine commitment to helping his friends. He creates the android Nana from a desk lamp, and through his relationships with Kuro, Dr. Go, Mi, and Nana, he evolves from a lonely boy into a confident and valuable member of the group. His growth sees him move beyond mere hero worship into becoming a resourceful ally whose intelligence proves just as critical as physical strength in the series' conflicts.