The Sigurd Institution is a secret research and warrior institute in the Vatican.
The Sigurd Institution was created with the the purpose of producing the [True Descendant of Sigurd]; someone capable of wielding the Demonic Emperor Sword Gram from among those who had inherited the blood of the hero Sigurd. The institution used a variety of genetic modification and artificial human creation techniques in their attempt to create the [True Descendant of Sigurd]. Their first creation Siegfried was a success as he was able to draw out and wield Gram in a single attempt. After the original goal of the research institution had been accomplished, the institute changed its goal afterwards into seeing how many descendants of the hero Sigurd the researches could produce.
While originally a research institute, due to joining with with other warrior institutes, the institution also functioned as a warrior institute, with it being used as location to train warriors. It was stated Tosca was a part of the warrior institute but because she didn't meet the requirements to be a warrior, she was sent to the Holy Sword Project.
|A promising stock of the Sigurd Institution. He greatly respects Siegfried and wishes to be Gram's new wielder.||Human||Alive|
|A member of the Hero Faction and one of the top two swordsmen of the Khaos Brigade alongside Arthur Pendragon of the Vali Team, who was known as The Demonic Sword User and "Chaos Edge" Sieg.||Human||Deceased|
|A Stray Exorcist who was once allied with the Fallen Angel Raynare, as well as Kokabiel and later became a member of the Khaos Brigade, allying with Diodora Astaroth.||Chimera (Former Human)||Deceased|
|An exorcist who is a member of the Vatican and Heaven's Brave Saints as an Extra Joker. Lint is a former member of Rias Gremory's Azazel Cup Rating Game team; Team Rias Gremory, filling in the spot Xenovia Quarta left as her Knight. She is the possessor of the Longinus; Incinerate Anthem.||Reincarnated Angel (Former Human)||Alive|
|An old friend of Yuuto Kiba and another survivor of the Church's Holy Sword Project. While originally belonging to the Sigurd Institution, she didn't meet the requirements to become a warrior, which is why she was sent to the Holy Sword Project.||Human||Alive|
- All of the members of the Sigurd Institution have white hair due to either being artificially produced from the original Siegfried's genes or undergoing the specialized warrior training. 
- The institution was a secret among the general sect of believers due to the heretical implication of trying to artificially produce living human beings.
- Yuuto Kiba is considered to be deeply connected to the institute due to several reasons:
- The first reason is that he defeated both Siegfried and Freed Sellzen, who were a part of the institute.
- The second reason was that he is the wielder of Gram, the sword which the institution was created in hopes of creating a wielder for it.
- The third reason is the fact that his companion Tosca was previously a part of the Sigurd Institution before becoming a test subject for the Holy Sword Project.