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Seto was a High Priest in ancient Egypt, and Pharaoh Atem's cousin. He owned the Millennium Rod and used it to try to defeat Atem and take over the world (or at least that's what it seemed). But he lost and seemed to die as a result. The battle is shown in the stone tablet Ishizu showed Seto (who refused to accept it), then Yami and Tea. But the Millennium Rod eventfully opened Seto's eyes to accept the truth, though he still has some doubts.

Also, Seto had a relation of sorts with Kisara, a 'foreign' young woman with fair skin and light long hair who became the Spirit of the Blue-Eyes White Dragon to protect Seto. Ironically, Priest Seto's issues with Atem and his feelings for Kisara carried on in Seto's intention to beat Yugi and his obsessive collecting of the three existing Blue-Eyes White Dragon cards.

In later episodes, during the duel between Seto and Yugi in Battle City, they were taken to their past memories. In the memories, Yami asked Priest Seto why has he joined the dark one and wants to duel him. Priest Seto says that he's not on anyone's side, he just want to defeat Yami (just like in the present). During this duel, Ishizu also said that Priest Seto strengthened Yami in battles, and was his friend as well as rival.

In the manga, in the past, during the battle (real battle!) between Yami and Thief King Baruka, Priest Seto also helped Yami. It is Priest Seto's father who wanted power for his son in the past (Priest Seto's father has the Millennium Eye), not Priest Seto (or Seto).

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