Koji Kabuto appears in different media related to Mazinger that is not canon to other series, often as the protagonist.
Koji Kabuto is one of the protagonists along with Devilman. At the start of the film, he fights against Mechanical Beasts with Mazinger Z. Later he encounters Devilman in the form of Akira Fudo, who insulted Mazinger since it couldn't fly. Koji was angry and challenged Akira to a motorcycle race which ended in a draw. Later when Sayaka and Shiro are kidnapped, Koji goes to save them on the Hover Pilder. During the attempted rescue, he saw Akira change into Devilman. Greatful for rescuing Sayaka and Shiro, Koji returns the favor by destroying the demons who captured Devilman with Mazinger Z now able to fly. With the fight over, the heroes fly into the sunset.
Koji is enjoying a relaxing day with the battle with Dr. Hell over, however his time to relax is cut short when Boss and his gang tell them of a prophet who informed them of an apocalypse where Mazinger is the only defense for. Koji thought Boss and his gang were hallucinating from the heat. But the prophecy came true with the attacks of the Warrior Beasts of the Mycenae Empire. Koji fights the monsters with the Mazinger, but they prove to be stronger than the Mechanical Beasts and the Ghost Mechanical Beasts that Gorgon gave to Dr. Hell, even with the disadvantage in environment Koji manages to destroy them but the Mazinger is too damaged to fly to the lab. On the way back, he is attacked by more Warrior Beasts and is badly wounded. But when he is about to give in, Sayaka informs Koji that Shiro's life is in danger, giving Koji the strength to defeat the Warrior Beasts.
Koji made it back to the lab where he learns that Shiro needs a blood transfusion. Koji offers his blood to help his brother, despite the others protests. Koji ignores them and the transfusion went underway. While Koji waits to hear about Shiro's condition he hears the voice of the prophet where he learns that the Mazinger will be destroyed if he fights again. Prof. Yumi recounts something about the Empire and Koji discovers from his grandfather's notes that the empire were waiting thousands of years to retake the surface. Even with the Mazinger still damaged and Koji not in the best condition, he decides to fight to the last of his breath for even a chance of survival. Sayaka gave him the present Shiro never could before he leaves.
Koji activates the Mazinger and fights against Juuma's Warrior Beasts. He manages to destroy a couple of them but is driven to a wall with Koji knocked unconscious. Koji is then awoken from his present, a music box just as Juuma is about to strike him down witnessing the Great Mazinger helping him. The Double Mazingers work together to destroy the remaining Warrior Beasts and Juuma himself. As Great's pilot Tetsuya introduces himself, Koji wonders who could have made the Great as it flies away.
Koji, Sayaka, Lori, Loru, and Dr. Morimori were on vacation in Paris after a fight that left the Mazinkaiser damaged and needed repairs. When having a meal with his friends, an image of the Great General of Darkness appeared announcing the conquest for the surface. Koji and co. left to get back to Japan to activate Mazinkaiser to fight back. Along the way Lori, Loru, and Dr. Morimori were killed by the Mycenae Army Generals while finishing them off. After getting back to Japan, he and Sayaka try to get back to the Photon Lab with help from a trucker named Riki. But Archduke Gorgon had chased them from their last encounter. As Sayaka lead him away, she was knocked out by Gorgon. When Koji went to her side, they were about to be attacked until Koji shot at him and Boss Borot appeared to throw Gorgon into the gas station which then exploded. Koji left Sayaka in Riki's care as he went with Boss and co. to the Photon Lab. The lab is in ruins but the Kaiser was moved into a satellite as the remaining Generals and Warrior Beasts arrive, Koji pilots the Kaiser Pilder as the Mazinkaiser drops through the atmosphere. After crashing, Kaiser activates and decimates the Mycenae Army. Soon the Great General arrives on the Demonika, Koji fights against the General and in the final stretch deals the finishing blow while yelling out the names of the people harmed by the Empire. The General is knocked into a valley where his true face is impaled by his own sword. With the battle over Koji joins the ceremony honoring his fallen comrades before having Kaiser pick up the Great Mazinger with Tetsuya and Jun.
Koji had returned triumphant with the defeat of Dr. Hell. But Dr. Hell was still alive and had gained control of the Devil Mazinger. Koji fought the early Mazinger but could not win. Later he discovered his grandfather's final robot that was made to fight the Devil Mazinger, Mazinkaiser. With this new robot, Koji fought Dr. Hell and managed to destroy him and the Devil Mazinger.
Koji had fought and defeated another Mechanical Beast daring Dr. Hell to throw whatever he could at him. Soon the Photon Lab is under attack by the Kedora who took control of Mazinger. Koji witnessed its rampage recalling his grandfather's words about how Mazinger could be a god or a devil. Koji gives chase on the Hover Pilder with Sayaka, but the Garadoubla MK01 appeared which made matters more complicated. Regardless, Koji managed to force the Kedora off which later caused it to take control of the Garadoubla. Koji fought the Mechanical Beast with Z and destroyed it.
Koji looks at the remains of the Mazinger now no longer able to fight. Sayaka shows him the upgraded Aphrodite A and says that she'll be ready to fight again. But Koji advises her not to as he wouldn't be there to protect her. Koji then rides off on his motorcycle where he encounters a different Baron Ashura who guides him to his leader, Dr. Heaven who is revealed to be the original Juzo Kabuto. Dr. Heaven reveals to his grandson that the Juzo who created Mazinger Z and Dr. Hell were actually clones of him that were created to help him research the Mechanical Gods from ancient times. Dr. Heaven then gives Koji a new robot to fight with, Mazinkaiser. With this new robot, Koji goes out ready to fight.
Koji is fighting the army of Pikadron robots with Mazinger and Great but they are outnumbered and overpowered when they started to absorb their energy. Koji protects Tetsuya from the robots when the Great Booster arrives, putting the Great on the attachment while Koji takes Great's Mazinger Blade. The Mazinger is then flung into the ocean by the robots. Koji was rescued by Hikaru Makiba of the Space Science Laboratory while Mazinger is left behind. During the battle against Gilgilgan, Koji arrives in the Double Spazer to save Shin Getter Robo before being instructed to go back to the Photon Lab where a new robot is ready for him. Koji receives a warm welcome from Sayaka and Shiro before being introduced to the Mazinkaiser that was made with the engines of Z and Great. Koji thought of Kaiser as the legacy of his grandfather and father becoming one, given a pilot suit by Sayaka Koji goes out into battle. Using Kaiser, Koji wipes out the enemy army and re-energizes the Shin Getter Robo. The super robots then fought Gilgilgan as he assumes his Mecha form. Thanks to the Super Alloy New Z alpha on Kaiser, Koji manages to make a plan to destroy Gilgilgan and his eggs after penetrating a hole, Kaiser and Shin Getter destroy Gilgilgan and the Invaders are defeated. With the battle over, Koji is confident that the power of the robots will defeat anything that comes their way.
Koji and Mazinger Z are the first to fight Varon and his Meteor Monsters getting help from Steel Jeeg. He later appears in a few scenes discussing events and with Mazinger when pairing up with the other Super Robots to fight Varon especially when using the convoy to become Hyper Robo Dynamic Saga. After Varon is destroyed the pilots and robot proclaim they will defeat any threat to Earth.
Koji appeared with Mazinger Z to help Sayaka when she was losing against the Mechanical Beast Zetsurion H69. Koji gloats on how easy it was to win compared to when Sayaka in Aphrodite A Neo fought it, which made Sayaka angrily walk off. When she does not come back to the lab or answer her phone, Koji goes with Boss to find her. But Koji only finds her cell phone before being called to fight a Mechanical Beast that appeared in the city. Koji gets Mazinger and finds the Mechanical Beast Gargle RD7 holding naked girls from his high school hostage, and one of them is Sayaka. Koji tries to fight without hurting the girls which becomes difficult when Baron Ashura flashes Sayaka's private parts at Koji. With all of these openings, Koji was thrashed about by Gargle. But in the smoke of an attack, Mazinger manages to free some of the girls with more help from Boss and Prof. Yumi who brought Aphrodite A Neo. With Sayaka liberated, she and Koji worked together to destroy Gargle. As Baron Ashura retreated, Koji tried to get a good look at Sayaka who was still naked. Sayaka tried to move away but was caught by Mazinger.
Koji is dispatched to fight the gigantic masked but nude woman Kekko Kamen. Koji fights her believing her to be an accomplice of Dr. Hell. Kekko merely fights against Mazinger, whose acrobatic fighting style and nude body distracted Koji causing him and Mazinger to be restrained. As Kekko explains that she is not an enemy, Koji gets a message saying that the Photon Lab is under attack with the Jet Scrander being sent to him. Kekko however highjacks the attachment with Mazinger following. Together they defeat Dr. Hell and his Mechanical Beasts. Koji just has Mazinger hold Kekko in its arms in victory.
Koji and Sayaka are sent to the future in a lab accident to a world ruled by sentient and war-loving robots called Creatures. They join a resistance composed of young women who provides Koji with a copy of Mazinger Z. Koji fights against the Creatures and later gets help from fighters of his time. But when the robot Gallahan reveals himself to be the Emperor of Darkness who took over Grendizer, he tries to escape back in time. While the Getter team deals with the other Getter robots Koji and the Mazinger team give chase. However, Dr. Hell had been revived as the Devil Mazinger having combined with a previous Mazinger robot. Koji and the others try to fight Hell and his new form but are beaten and the copy Z is destroyed. Koji wants to act but can't do much without a robot or a way to fight the Devil Mazinger. Luckily, the answer comes into the new form of the present Mazinger, the God Mazinger. With this new robot, Koji fight Dr. Hell one-on-one while the others deal with his army. As God Mazinger manages to counter the Devil Mazinger's energy absorbing ability, Koji thrusts the robots sword through the face on its chest destroying the Devil Mazinger and Dr. Hell. With the battle over, the others worry about the future while Koji is confident it can be changed.