Seth set it up so once he was done as pharaoh he would reincarnate at the time when the seal against the Shadow Realms began to crack so he could again help the pharaoh defeat Zorc and help restore the balance between the Shadow Realm and earth.
He was reborn as Seto when the seal began to weaken. His life was fairly happy. He had a younger brother Mokuba, four years younger than him. When Seto was eight his parents died in a car crash moving them to an orphanage. Seto refused to let their next family be chosen by chance and possibly separate him from his little brother. Seto was a genius, with a photographic memory and a brilliant strategic mind. He used his talents when he was nine to force the multimillionaire Gozaburo Kaiba to adopt him and Mokuba by defeating him in a chess game.
Gozaburo took the boys reluctantly. He decided to mold Seto into the perfect heir as he had no choice and needed to find a use for the brat. Seto was bright enough. Mokuba was for the most part ignored Gozaburo threatening to turn his attention to him if Seto did not perform as Gozaburo demanded. Seto was abused, kept up without a chance to sleep to study and work in Gozaburo's weapons company, whipped if he showed himself as anything less than perfect in his actions. The dream Seto had shared with Mokuba of one day opening amusement parks around the world free for orphans had come to an end.
Mokuba tried to support his brother. He learned of his brother's love of Duel Monsters and made a Blue Eyes White Dragon for Seto since they couldn't have the real thing. In the rare moments that Seto could escape Gozaburo they could pretend they were still together and would play. Seto decided to make this home for him and Mokuba. It wasn't ideal, but he wouldn't be Gozaburo's puppet. He schemed, hardened himself, and finally when 12 beat Gozaburo at his own game with Mokuba's help.
Gozaburo was no longer part of their lives, having died from jumping out a window after being defeated by Seto. The scars from Gozaburo's care remained. He was cold and aloof. He couldn't connect to his brother the way he used to, and shunned anything that could be seen as weakness. His brother was protected in the world Seto had created, but it was an empty one. Even when Seto changed it from a weapons company to a toy company and began plans for the amusement park they'd once dreamed of Seto remained distant and cold.
He became obsessed with winning. He would never lose again. He would never be that vulnerable. Mokuba was his only weak spot, and even his brother had trouble approaching the aloof businessman. Seto began to collect the real Blue Eyes White Dragon cards, forgetting that his first one had come from Mokuba. He received three of the four cards made of it, and began his search for the fourth, unable to be happy until the dragon belonged to only him.