Saiji Tatemiya leads the Amakusa-Style Remix of Church as Substitute Supreme Pontiff, stepping in after Kanzaki Kaori's departure and willingly relinquishing leadership upon her return. He possesses a tall, slender frame and wears loose clothing, including an oversized shirt displaying a slanted red cross (St. George's Cross, symbolizing the Amakusa's desire to join the Anglican Church). His pitch-black hair resembles a beetle's carapace, and he sports basketball shoes with one-meter laces and a rope necklace holding four to five 10-centimeter mini-fans. Every clothing item functions as a deliberate component of his magic arsenal. His personality demonstrates deep loyalty to comrades and strict adherence to Kanzaki's principle of aiding the abandoned. He prioritizes freeing captured allies over personal escape and exhibits intense rage when they are harmed. He actively supports their interests, such as encouraging Itsuwa's relationship with Kamijou Touma and intervening to reject the Knight Leader's advances toward Kanzaki. He disapproves of self-pity among his followers, compelling Itsuwa to repay Touma for saving her from Acqua of the Back and helping her regain composure after learning Touma survived World War III. In battle, he displays confidence, sarcasm, and strategic seriousness. His background involves leading the Amakusa during Kanzaki's absence, driven by collective guilt over her prior defection. The group sought strength while upholding her tenets, notably offering refuge to those in need like Orsola Aquinas. He may have participated in events shaping Kanzaki's past, including the creation of the Saint Killer spell to understand her capabilities. Key chronological involvements include the Orsola Aquinas Rescue Arc, where he was initially misidentified as an antagonist while pursuing Orsola to protect her from Roman Catholic assassination. After confrontation and temporary capture by Touma and Stiyl Magnus, he revealed the Roman Church's true intentions, freed his comrades, and coordinated with Touma's group to rescue Orsola, deploying Stiyl's rune cards to summon Innocentius against the Agnese Forces. During the La Regina del Mare Adriatico Arc, he directed the Amakusa in relocating Orsola to England. Following the ambush of Touma and Orsola, he led a rescue using wooden submersibles to retrieve them from the Adriatic Sea and orchestrated a counterattack against the Queen's Fleet with fire ships, enabling Touma and Orsola to infiltrate the flagship and free Agnese Sanctis. In the Acqua of the Back Arc, he reprimanded Itsuwa for failing to protect Touma during Acqua's assault, later coordinating the Amakusa's battle against Acqua. They employed Body Enhancement Magic for hypersonic mobility. Though initially overwhelmed, the group assisted Kanzaki in neutralizing Acqua's dive-bombing attack, facilitating Touma's intervention and Itsuwa's successful Saint Breakout technique. His combat abilities center on Amakusa-style magic, utilizing everyday objects for spells. Specializations include barrier/defensive magic, Anti-Impact deflection, fire resistance, and Forge Magic to enhance his 180-centimeter Flamberge sword. Group-exclusive techniques like Body Enhancement Magic allow hypersonic movement through coordinated back-touching formations. He also employs healing magic. Relationships highlight his role as a unifying leader: he treats the Amakusa as Kanzaki's "family," supports Itsuwa's emotional growth, and fosters alliances with Touma and Anglican figures like Orsola Aquinas.