When Caswell asked him to help Yuma find Emperor's Key using the Crashbug Virus he created while he had a "Number", he was confused about this, but then accepted when Caswell blackmailed him with the threat of telling everyone that he was responsible for the Crashbug Virus incident. Using the virus, he located the Key and opened the door to the building where it was held for Yuma to go retrieve it. He was able to override the building's computer network with his Crashbug virus, causing it to exploded.
He made another non-speaking cameo in Yuma's class during the voting for class representative. Mr. Kay was among the survivors of Heartland who did not pick up a "Fake Number" and be sacrificed to the Barian World.
Mr. Kay uses a "Crashbug" Deck revolving around Summoning his "Number", "Number 34: Terror-Byte". Much like the effects of his cards, his Deck represents what he has done - hacking into computer systems and taking control of them, launching "bugs" en masse, and wroughting many people (and the infected systems themselves) trouble because of it all.