Takuma Saiou or Sartorius (4Kids) is the main antagonist of the second season of Yu-Gi-Oh! GX. He is a fortune-teller with dual personalities. Being under the influence of an alien force, Saiou is poised to remake the world in the image most befitting to the extraterrestrial radiance within him.
In the past, Saiou and his sister Mizuchi Saiou were able to tell the future. They were embarrassed by others and ran away because of their powers. Saiou had later met Edo Pheonix and had a friendly relationship with him. At the Pro League champion, Saiou was taken over by the Light of Destruction when DD came into contact with him. From then on, Saiou became focused on changing the world and ridding it of darkness. He also formed the Society of Light in his efforts to expedite his plans.
Saiou appeared on Duel Academy in his hopes of completing his plans. He first planned to have Judai lose to Edo in their second match which worked so that Judai would be placed under his control. However, this did not work out in Saiuo's favor as Judai was able to defy the Light of Destruction. Saiou later duels Jun Manjoume and defeats him. As a result of the defeat, Jun had joined the Society of Light.
Saiou later finds out that Judai is able to defy his destiny and the fortunes when he thought that Judai would lose in a duel to Edo. The members of the Society of Light began to increase subsequently when Jun were beating the students in their matches. He was interested in Jun and his ability to defy his own destiny. He later dueled Tyranno Kenzan and was interested to see the bond he shared between Edo and Judai. Although he defeated Tyranno in a duel, he did not become a member of the Society of Light.
Later on, Saiou moved into the Obelisk Blue Dorm. His sister, Mizuchi also appeared and she talked about their troubled past and Saiou's turn to the dark side. As the Society of Light began to expand, Saiou had trouble controlling himself against the influence of the Light of Destruction. He also defeated Prince Ojin in a duel and gained access on a satellite that would help him in his plan to take over the world. He later gave Judai and Edo the keys as a way to prevent himself from carrying out his goal. He also got a hold of Asuka Tenjoin and used her in an effort to get the key back from Judai but it failed. Fed up by this loss, Saiou ordered many members from the Society of Light to duel Judai and anyone else who were with him.
Edo wanted to go after Saiou. But when he was about to give Saiou one of the keys to the satellite, Judai appeared and told him the truth about Saiou. He advised him on dueling him instead to get the key. However, Edo lost the duel and Saiou gained the key. Judai then duels Saiou in a final battle to save the world. As the duel went on, Saiou ordered Ojin to prepare the satellite. Ultimately, Saiou lost his duel to Judai and he was released from the influence of the light. With the defeat of the Saiou, the Light of Society was disbanded and all their members were released from the Light of Destruction's control. He later thanked Judai and Edo for helping him get pass his pain and reuniting him with his sister. Saiou and his sister then left the island.
Saiou later returned to the Duel Academy with Kagemaru to inform Judai about the new issue that awaits them on the island. They were caught of guard by Mr. T who blackmailed Saiou to duel Judai to save his sister from the World of Darkness. As he dueled Judai, he reflects on his own responsibility to save his sister from the World of Darkness. He lost in his duel against Judai and was sent to the World of Darkness. With the defeat of Nightshroud, Saiou and everyone who were sent to the World of Darkness returned. At the end of the series, Saiou attended Judai's graduation.
- Saiou is similar to Dartz from Yu-Gi-Oh!, they were both manipulated by a force of evil. Saiou was used by the Light of Destruction and Dartz was corrupted by Orichalcos.
- Saiou s similar to Saix from the Kingdom Hearts franchise, as they become deranged when exposed to a certain light (Saiou: The Light of Destruction, Saix: the moon).
- In the 4Kids dubbed version, Saiou wanted to brainwash everyone with the light so that he can rid them of corruption. In the original Japanese version, Saiou wanted to kill off Earth's inhabitants with the light so that he could restore balance to the universe as a whole.