Mr. Kay, known as Ukyo Kitano in the Japanese version, is a teacher at Heartland Academy where he instructs the class attended by the protagonist Yuma Tsukumo. Physically, he has wavy brown hair and typically wears brown pants and a matching vest over a pink collared shirt. His personality is presented as charismatic and dedicated to his students, though he is not a central figure in the overarching plot. His motivation as an educator is to guide and support his pupils, and he appears genuinely invested in their well-being.

In the story, Mr. Kay plays a supporting role that becomes notable during the early episodes of the series when he falls under the influence of a Number card. Specifically, he is possessed by Number 34: Terror-Byte, a powerful entity that takes control of his actions. This possession leads to a duel against Yuma, during which Mr. Kay uses a deck themed around insect- and virus-like monsters, reflecting Terror-Bytes corrupted influence. The encounter highlights how ordinary individuals can be exploited by the Numbers and showcases Yumas resourcefulness in freeing others from their influence. After the possession is broken, Mr. Kay returns to his normal life as a teacher, showing no further significant character development or major role in subsequent story arcs.

His key relationship is with Yuma, both as a student-teacher bond and as an adversary during the Terror-Byte incident. He also interacts with other students of Heartland Academy, but these relationships are not explored deeply. As a duelist, Mr. Kay does not demonstrate exceptional skill before his possession; his ability during the possessed state is derived from Terror-Bytes power rather than his own talent. Overall, Mr. Kay serves as an early example of the threat posed by the Numbers and reinforces the theme that seemingly ordinary people can be caught up in the conflict surrounding Yuma and the Number cards.