Saiji Tatemiya is a supporting protagonist in the light novel and anime series A Certain Magical Index. He is the Substitute Supreme Pontiff of the Japanese-based Amakusa-Style Remix of Church, leading the sect in behalf of Kaori Kanzaki who has vacated the position of Supreme Pontiff before the start of the series. As Substitute Pontiff he and the Amakusa Christians went on missions which carried the ideals that their Priestess (Kaori) believed in, and that is to always aid those in need. He later willingly steps down from the position after Kaori returns as the Supreme Pontiff of the Amakusa Church.
He is voiced by Kosuke Terumi in the Japanese version of the anime and Duncan Brannan in the English version.
Saiji is described as being around 25 years old with a tall and skinny figure. He wears very loose clothes, an over-sized shirt with a slanted red cross emblazoned upon it. His hair is pitch black and is implied that he used hair dye on it, and is described as something akin to a carapace of a beetle. As with its style, it is like Touma, except more messy. He wears basketball shoes with 1 meter long shoelaces. He also wears a rope hanging around his neck, where four to five mini-fans about 10 centimetres big are hanged.
As being part of Amakusa, all of what Saiji wears is part of his magic and is deliberate. With the St. George's Cross he wears it to signify that he wants to join with Kanzaki Kaori, their Priestess, with the Anglican Church.
To allies, Saiji is an amicable person who has shown to care deeply for his comrades, preferring to attack the Agnese Forces alone to free his comrades rather than escape without them. Moreover, he gets enraged when someone hurts them, as shown when Touma reveals that he has defeated Uragami. In pursuits of their interests, Saiji will gladly aid them, as shown when he and the others try to help Itsuwa become closer to Touma, and even shooed away the Knight Leader's advances on Kanzaki.
As much has he cares for his comrades, Saiji has shown distaste for them feeling sorry for themselves and being in unease, as shown when he forced Itsuwa to pay her debt with Touma saving her from Acqua of the Back, as well as aiding Itsuwa get a hold of herself after discovering Touma being alive after believing he was dead after World War III. Like Touma, Saiji will often come to aid of people he doesn't even know, stating that he doesn't need a reason to save someone.
As the substitute leader of the Amakusa Church, Saiji closely follows the tenets set down by Kaori, specifically, to reach out to all those that are abandoned. When battling, he is shown to be boastful and confident. He often makes swaggers and sarcastic comments during them, but that does not entail that he is not serious, as shown during the Battle of the Church of Orsola.