|Age||16 (Start of series), later 17,18 (Great Mazinger), 19-20 (Grendizer)|
|Family and Relations||Unknown|
Koji was born to Kenzo and an unnamed mother, after Shiro was born, his parents were killed in an accident, some years later, Juzo spent too much time in his Kabuto Manor, after getting a phone call at Shiro's school he races to the manor to find Juzo dying and he gives him Mazinger Z.
Koji has an athletic build, normal height and weight of a teen his age, black spiked hair and his trademark sideburns.
Sayaka and Gennosuke YumiEdit
Koji met Sayaka as he tried to pilot Mazinger for the first time, eventually, their relationship grew and they had mutual feelings toward each other, even though they got mad and fought, even though Koji doesn't want Sayaka to fight alone. Prof. Yumi cared for Koji in battle and be let Koji and Sayaka date.
When they first met, Tetsuya was jealous and did not try to help Koji, but Tetsuya was sorry and helped Koji.
Although Duke originally saw Koji as a naive nuisance and their rivalry, they became best friends.