Sigui is an inquisitor, a nun belonging to the Pope’s central office. Burning at the stake all devil worshippers, pagans, and even believers who show misconduct is her mission.In the OVA prologue, it is revealed that prior to her title as Inquisitor, she was stationed at the 16th monastery, and was invited by the Holy Dynasty to worship in the Grand Cathedral. During that time, she met Ymir following a skirmish with some drunken women, and has grown acquainted to her despite the latter's merchant-like personality. It is also shown that she tends to rush with her Holy Poses, often rendering them ineffective. She’s very eager to burn down the Rebel Army, which she considers to be a den of heretics. She also considers the use of alchemy as a heresy (even the Holy Dynasty has divided opinions on this topic). She is armed with the Holy Flame Mace and God's Chained Blade, and her fighting skills rivals Annelotte's. Like Melpha in Queen's Blade, she is also a user of the "Divine Power", the manifestation of God's miracles utilized by the provocative "Holy Poses". Later on, she has a crisis of faith, but Annelotte helps her and tells her she doesn't have to fight alone anymore. She ends up joining the Rebel Army, where she calls Annelotte "Elder Sister"(despite being older than her).
Sigui wears a very revealing take on a nun's outfit, wearing the traditional headpiece and neckpiece. On her torso she has an open, white, frilled dress, with a blue jacket on top of it with elegant designs and armbands. On her lower body she wears white garters underneath her metal thigh-high boots.
Sigui is a passionate devotee to God and her strictness is second to none, even to Queen Claudette. She has been given the authority to punish anyone that can be called heretic. It’s not that she's cruel, actually she’s usually virtuous, but primarily she's too serious and inflexible and doesn't hesitate in judging people.
Sigui fights with The Staff of Holy Fire, and also attacks with her other weapon, God's Chained Blade. With these weapons, she is able to channel extremely potent blasts of purple fire, which deal significant damage if hit directly. She’s also a user of the “Divine Power,” which Melpha also uses, which she uses to perform the Holy Poses.