Yuya's father disappeared in front of him. When he was most helpless, Nilin appeared in front of Yuya and taught Yuya the story of duel. It is set that Yuya and Nilin are together to form a complete Zack. Nilin is Zach's dark side/inner personality, with his memory and power, while Yuya is a quarter of Zack's light side/surface personality, which will later become a complete light side/surface personality, and all four dragons will appear in his hands.