The historical figure he portrays has a mythos that makes it seem as though he lived to the age of 106, and thus it is difficult to know if the person, historically or not, truly was still alive during the setting of the game.
Seto and his 7 followers, the Rose Crusaders served Pegasus Crawford, a powerful Yorkist nobleman. Seto almost had the Lancastrians at the mercy of the Yorkists. With defeat likely for the Lancastrians, Margaret Mai Beaufort requested a high druid, Simon McMooran, to summon a duelist from another age.
After Simon summons a duelist, (the player) Seto interrupts and begins to argue with Simon. Seto tells the duelist that they need 16 Rose Cards to return to their time and both sides have 8 cards, so they may side with either the Red Roses, the Lancastrians or the White Roses, the Yorkists. The player is then left to decide which side to join.
If the Player joins the Lancastrians
Seto is disappointed, but respects their decision.
On August 22nd, 1485, the final battle of the "Wars of the Roses" takes place and the Lancastrians emerge victorious. Henry Tudor thanks the duelist and asks him to duel Seto at Stonehenge to finally end his evil threat.
After losing Seto says that it was a duel to remember and that he is proud to have fallen at that duelists hands. But then Seto proclaims, it would take a fool to believe that, that he was truly defeated. Thanks to the duelist he could now complete the ritual for the Great Summoning of the Roses. He explains that he had planned all this from the start. He needed the Rose Duelist to perform the Great Summoning. He came to England and joined the Yorkists, knowing that the Lancastrians would resort to asking the high druid to summon the only duelist with power equal to his. A high powered duel involving the Rose Duelist, while all 16 Rose cards were at this location, was the Ritual. So regardless of his loss Seto was able to Summon the Rose Guardian, Manawyddan fab Llyr, who then duelled the player.
If the Player joins the Yorkists
Seto tells the duelist he wishes he could join him in acquiring the Red Rose cards off the Lancastrians, but he is needed to maintain their defenses. After the duelist defeats Henry Tudor (Yugi), the Yorkists emerge victorious and Seto congratulates the duelist on his victory. Seto then uses the Red Rose cards that the duelist obtains to complete the summoning, but only to accidentally summon Manawyddan fab Llyr. The Duelist then defeats Manawyddan fab Llyr in a duel. Afterwards, Seto apologises for the trouble he put the Rose Duelist through, giving the duelist a pendant, saying to show to his decendants in his/her time if they are in trouble, pledging his family's loyalty to the poweful duelist before reversing the spell and sending the Duelist home.
Seto plays a Deck mainly based on Seto Kaiba's containing many of his cards from the Duelist Kingdom arc. His Spell Cards focus on increasing the ATK of his monsters and his Trap Cards deal with negating harmful moves dealt by his opponent.