Kanji Yoshikawa is a classmate and friend of Jun Ushiro, and he becomes one of the fifteen children who pilot the giant robot Zearth. He is depicted as a boy with brown hair and brown eyes. Among the group, Kanji stands out for his intelligence, which is said to rival that of Kunihiko Moji, and his analytical nature often leads him to critique the strategic choices made by his fellow pilots.

His background and personal history are closely tied to the Ushiro family. Kanji was aware of the abuse Jun inflicted upon his younger sister, Kana, but chose not to intervene. He later reveals to Jun that his inaction was not due to indifference but because Kana herself had asked him not to get involved. In the anime adaptation, Kanji's role is significantly expanded. He is portrayed as the son of Mitsue Yoshikawa, a lead researcher tasked with understanding the alien technology behind Zearth. In this version, he also becomes emotionally involved with Anko, providing her with crucial support after her family is destroyed by a scandal. His encouragement gives her the strength to pilot Zearth and defeat her enemy.

Kanji’s motivations evolve from that of a detached observer to a decisive, if reluctant, participant. Initially, he is part of the group that signed the contract with Koyemshi, the creature that manages the battles, without fully grasping the fatal consequences. When his turn to pilot comes, his strategic mind is put to the ultimate test. His opponent is one of the strangest the group faces; it launches itself into the sky and lands in Hawaii, thousands of kilometers away, before firing powerful shots at a cannon orbiting Earth, which redirects the attacks toward Zearth. After the Japanese military fails to destroy the enemy with a nuclear bomb, Jun suggests that Kanji use the giant robot's lasers. Since Kanji cannot target the enemy directly, a human soul is required to act as a targeting marker. A military officer named Seki volunteers, and he and twenty-two soldiers are teleported to the enemy's location, allowing Kanji to fire and secure a victory.

Key relationships define much of his personal journey. His friendship with Jun is central, particularly in the manga where Jun reveals he is not bound by the piloting contract. Jun admits that his decision to stay with the group was not solely due to guilt over a classmate's death, but also for Kanji himself, an admission that deeply moves him. His relationship with Kana is also complex; his promise to her kept him silent about the abuse she suffered. In the anime, his bond with Anko highlights his capacity for empathy and support, even as he is forced to confront the tragedy of the situation. Koyemshi initially intended for Kanji to be the pilot for the final battle, but after serious discussions, particularly once Kanji discovered Koyemshi's true manipulative nature, the creature changed his mind and scheduled his fight earlier.

Over the course of the story, Kanji develops from a clever but somewhat detached observer into a young man who fully grasps the weight of his duty. He experiences fear and hesitation, especially when confronted with an enemy that fights from an impossible distance. However, he ultimately shows immense resolve, taking decisive action to defeat his opponent. After his victory, instead of despairing, he requests to be teleported to the top of a tower, where he spends his final moments contemplating who will be the next pilot to sacrifice their life. His notable abilities are not superhuman but intellectual; his primary strength lies in his sharp tactical mind, which he uses to analyze combat situations and devise unconventional strategies to overcome seemingly impossible odds.