Korosuke is a robot samurai and the first invention created by the young genius Eiichi Kite. He was constructed based on the blueprint for a karakuri doll left behind by Kiteretsu's ancestor, Kiteretsu Saisama, who originally designed the robot to resemble his own deceased son. Built from repurposed household items such as a rubber ball for his head, a washbasin for his torso, and vacuum cleaner hoses for his arms, Korosuke is a fully functional automaton with human-like emotions, consciousness, and the ability to speak.

In terms of personality, Korosuke has the mentality of a young child. He is playful, often selfish, and can be quite mischievous. He speaks in an archaic, self-important manner typical of a samurai, referring to himself as Wagahai and ending his sentences with the particle nari. Despite his bravado, he is very emotional and sensitive. He is easily frightened by barking dogs and is prone to tantrums when things do not go his way. He has a strong sweet tooth and a notorious love for croquettes and cake. He is deeply loyal to Kiteretsu, whom he sees as both his master and his best friend. He struggles with keeping secrets and is a terrible liar, which often gets the group into amusing situations.

His primary motivation is to be a good and helpful companion. He craves recognition and inclusion, often feeling jealous or lonely when Kiteretsu and his human friends go to school or exclude him from other activities. As the story progresses, his motivations evolve from simple selfish desires to a genuine wish to care for and protect those he loves, particularly his creator.

Korosuke serves as Eiichi Kite's constant companion and assistant. He is an integral part of the friend group, joining Kiteretsu, Miyoko, Tongari, and Butagorira on their numerous time-traveling and scientific adventures. His naive and often clumsy nature provides significant comic relief, but his courage and loyalty also make him a reliable partner in moments of crisis. His presence is the emotional core of the many episodic adventures.

His relationships shape the heart of the series. His bond with Kiteretsu is the central pillar, a deep friendship between a creator and his creation that is tested and strengthened over time. He is very close with Miyoko, who maternally dotes on him, calling him Koro-chan. His relationship with Butagorira and Tongari is more adversarial; they often tease and bully him, but there is a clear underlying friendship and mutual respect. The most profound relationship is with Kiteretsu Saisama, the original designer of his blueprints, whom he comes to revere as a father figure.

Throughout the series, Korosuke undergoes significant character development. He starts as a very self-centered robot but gradually learns the value of cooperation, sacrifice, and genuine altruism. The climax of his emotional development occurs in the anime finale, where he makes the difficult decision to leave the modern era and remain in the Edo period to care for the lonely and aging Kiteretsu Saisama, demonstrating a mature sense of duty, independence, and love that marks his departure from childish innocence.

Korosuke possesses several notable abilities. He is surprisingly strong and agile, capable of performing gymnastic feats and wielding the small katana he keeps strapped to his back with real skill. His vision is extraordinarily sharp, and he has a keen sense of hearing. Although he can eat and digest food just like a human, his primary power source is a spring motor that requires regular winding. He is functionally durable as a robot and is a competent fighter in his own right, though his small stature and timid nature often hold him back in direct confrontations.