Relationships
- Shiro Kabuto (Kabuto Shiro): Kouji's younger brother. Kouji is very protective of Shiro.
- Sayaka Yumi (Yumi Sayaka): Kouji's partner and girlfriend. While tomboyish and sometimes violent, she and Kouji are close. Unfortunately, Kouji's tendency to rush into situations and speak without thinking often irritates her, resulting in a bop to Koji's head. She pilots the robot Aphrodite A, and later Dianan Ace (both infamous for beginning the trend of breast missiles).
- Boss: Originally Kouji's rival, Boss eventually had his own robot Boss Borot built literally out of junk. While at first antagonistic, the two became good friends and Boss and the Borot often helped out in battle.
- Tetsuya Tsurugi (Tsurugi Tetsuya): Though they're technically adopted siblings (Kouji is the biological son of Kenzo Kabuto, Tetsuya's adoptive father), they don't meet in person until the end of Mazinger Z. They get along well in their first meeting, but when Kouji returns with Sayaka from the USA during Great Mazinger, Tetsuya develops a strong jealousy towards him, and isn't able to fully get over it almost until the end.
- Daisuke Umon/Duke Fleed (Duku Fureedo): The hero of Grendizer, he was exiled from planet Fleed, fled to Earth and became the adopted son of Professor Umon, a leading scientist with whom Koji starts to work with at the beginning of the series. Daisuke and Kouji become friends, but there is an underlying tension in the relationship and at times they even come close to blows. For much of the series, Duke Fleed, who is prepared to risk his life but not that of others, tries to discourage Koji from joining him in battle against the alien Vegan, though he appreciates him as a potential replacement should he himself be killed. While Duke pilots the main Grendizer unit, Kouji pilots the Double Spacer that allows it to remain airborne for longer periods of time.
- Hikaru Makiba: Daughter of the owner of a ranch in the area where Grendizer is based and where Daisuke and Duke work in their spare time. Originally just a normal farmgirl, she soon proves to be a talented pilot and joins the battle against the Vegans. Koji falls for her at this stage, but she is in love with Duke Fleed, and has been so for some time. This causes some tension between the trio but a good fight with the Vegans enables Kouji to work it out of his system and they are soon friends again.
- Maria Grace Fleed: Duke Fleed's tomboyish younger sister, raised by her family's butler on Earth after the destruction of their native planet. She appears late in the series and falls in love with Kouji; it is hinted he is attracted to her as well even though they never stop arguing, but at the end of the series both she and Duke return to space to rebuild their homeworld. It's a running gag in many Super Robot Wars games as well in the Dynamic Heroes e-click manga that Sayaka and Maria are rivals for the oblivious Kouji's attention.
- Juzo Kabuto: In Shin Mazinger, Kouji is very much dedicated to his grandfather who took him and Shiro in when their parents died. His dedication is to the point that his normal, calm exterior is only cracked if his grandfather is threatened in any way. When the elder Kabuto is killed during their first encounter with Baron Ashura, Kouji is pushed over the edge and it takes the effort of the combined Mazinger Army and Aphrodite A to stop him.
- Tsubasa Nishikiori (Nishikiori Tsubasa): Originally known as a close friend of Dr. Juzo Kabuto, Tsubasa takes Kouji and Shiro in after their grandfather's death. While thankful for having a place to stay, Kouji initially chafes under her tutelage. Eventually, he comes to realize her good intentions after noting his improving skill in piloting Mazinger. She is later revealed to be the mother of Kouji and Shiro and the wife of Kenzo Kabuto.
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