Koji Yuki is the father of Hayden Yuki and Judai Yuki. He's also the husband to Arisa Yuki. An Asian born resident of Japan, Koji transferred to the states of New York to study abroad where he'd met Arisa. Though initally disliked by her, he became intrigued because she was generally the only person that would actually talk to him, even if it was rudely or occasionally threaten to beat the hell out of him if he didn't go away. He'd fallen in love with her as a result of his own curiosity about her and even saved her from being beaten to death by her own gang members. After that, she'd fallen in love with him. The two of them married later right out of high school and had their children. They moved at least twice before settling in Tokyo, Japan where Koji had gotten a job offer to work at Todai University as a Foreign Language teacher. He also used to play the game of duel monsters, still does once in a blue moon but with a newer deck as he gave his old deck of Elemental Heroes to his son Judai. Generally, Koji is an all around nice person and willingly befriends anyone he meets. He also has a very large appetite when it comes to food (something that Judai inherited from him), and is actually scared of his wife when she goes into one of her fits of rage when something upsets her and is normally the one trying to calm her down with not much success.
- Koji is actually scared of his wife but despite that he loves her dearly, faults and all.
- Koji is the one that Judai inherited his love of food from.
- It can be assumed that Hayden inherited her father's gentle personality.
- Koji is a former duelist. He has a newer deck that has not yet been known and only uses it against his son whenever Judai's bored and wants something to do.
- He gave Judai his Elemental Heroes deck.