Seto is a priest who is a cousin of Pharaoh Atem. Seto Kaiba is the reincarnation of Priest Seto. Seto is named after the Egyptian God Set. Priest Seto and Pharaoh Atem were friends and rivals who helped each other by testing their skills (just like Yugi and Kaiba did); as children their relationship was Atem as a prince and Seto as a priest on training. Source: http://yugioh.wikia.com/wiki/Priest_Seto