Yu-Gi-Oh! GX episode 118: "A Snake in the Grass, Part 2"
Pierce, known in Japan as Rick is Viper's adopted son. Viper saved him from being hit from a bomb when he was a baby in a war zone area and decided to adopt him. (In the Japanese version, Viper says that Pierce “was an angel who came to save his life”, as that same bomb took out his team and would've done him in, as well, had he not spotted the baby.) Years later, Viper gave him a deck of Duel Monsters cards but a sudden gust of wind blew the cards onto the street. Pierce went after them only to get killed by an incoming bus heading in his direction (the English version changes his death to having been due to a fatal disease). Since then, Viper had made a deal with a devil arm he found in a later mission, agreeing to help it in exchange for reviving Pierce.
Once Viper and Jaden’s Duel is over, the mysterious devil arm takes the plasma form of Pierce and changes Viper’s memory so that he believes Pierce is alive, thus carrying out their "contract".
Pierce gets killed when a bus hits him is similar to Noah Kaiba when he got hit by a car, and in both English versions they die from a disease.