Koyo Hibiki is the younger brother of Midori, and once a three-time Duel Monsters champion. Koyo was a great Pro League Duelist who used "Elemental Heroes". Chazz received an autograph from him, and in the process, Koyo's "Winged Kuriboh" altered Chazz's "Light and Darkness Dragon" card. However after losing a Shadow Game to a mysterious Duelist, he found that every time he drew a card while dueling seriously, the Deck of his heart would be depleted, at which point he would die. The Duel was erased from his memory. Koyo suffered from this and was sent to the hospital where he met a young Jaden who had broken his leg while playing baseball. He taught Jaden all he knew about Dueling and to always have fun Dueling no matter what. With his health failing, he decided to give up Pro-Dueling and made Jaden his final opponent. He beat Jaden in the end, by a blow from "Elemental Hero Terra Firma". After defeating Jaden, he gave him his Deck and his lucky red jacket. However, he found that in this he had drawn the last card, completing the curse and falling into a permanent coma, presumably kept alive by Winged Kuriboh.
He is revived after Jaden and Chazz defeated Tragoedia, and faced Jaden in a Duel once more upon the conclusion of the manga.
Koyo's full name means "autumn colors", but can also be translated to "autumn sounds".
Koyo plays an nature-based "Elemental Hero" Deck. Before the start of the manga, Koyo gave Jaden his Deck, including the one-of-a-kind "Elemental Hero Terra Firma" card.
The list given here are only the cards he was actually seen using. Jaden has used many other cards that could be assumed to have formerly been his, but it's also clarified that Jaden modified the Deck since receiving it.