The Three Priestesses (三神官 San Shinkan) are three ancient priestesses of the Eastern Watchdog Castle, composed of Keiru (ケイル Keiru), Beru (ベル Beru), and Rosu (ローズ Rōzu) Keiru, Beru, and Rosu are portrayed/voiced by Keaki Watanabe (渡辺 けあき Watanabe Keaki)/Fumiko Inoue (井上 富美子 Inoue Fumiko), Anri Okamoto (岡本 杏理 Okamoto Anri)/Machiko Kawana (川名 真知子 Kawana Machiko), and Yukina Kashiwa (柏 幸奈 Kashiwa Yukina)/ Akemi Satō (佐藤 朱 Satō Akemi). Their combined form Gulm (ガルム Garumu) is portrayed by Kimika Yoshino (吉野 公佳 Yoshino Kimika).
As Jabi revealed to Kouga Saezima, the Three Priestess were originally a single being that was among those chosen to become a link between human world and the Makai a long time ago. But overtime, though made immortal, the physical form of the that Watchdog wasted away as her spirit continued in three splintered and aged manifestations that assumed the form of little girls in white angelic clothing with their "son" Kodama for company. Because they were originally one, the Three Priestesses' thoughts are so unified that they sometimes complete each other's sentences in conversations.
At first, the Three Priestess carry out their expected duties for the Saezima family. But they are revealed to have been supporting Barago for the common goal of unleashing Messiah into the world. Though their reasons are unknown, the priestesses misused Horror and Madou materials to silence those that interfere with their plans like Amon and use Rei Suzumura, who they deceived into doing their dirty work, and Yūki Kamisugawa, by revealing the circumstances of his daughter's death, to kill Kouga.
When the time of Messiah's arrival draws near, the Priestesses' spiritual essences merge after Barago provides them a female corpse to inhabit so they assist him as Gulm. Gulm personally attack Kouga and Rei after the former killed Kodama, with Rei attempting to hold her off. Though Gulm was the stronger opponent, her overconfidence of the situation left her open to be impaled by Rei. After Gulm's death, Jiiru is stationed to oversee the eastern territories.
Gulm displays an ability to transform into a Cerberus-style armored form called Beast Gulm (獣化ガルム Jūka Garumu) form. She fights using an arc and her fighting techniques that are a stronger version of Kodamas'.
- The names of Gulm and her splintered form reference hellhounds for European mythology that both guard the gates to the realm of the death: Keiru, Beru, and Rosu named after the three-headed Cerberus (ケベローズ Keberōzu) of Greek myth and Gulm after Garm of Norse myth.
- It is unknown if Gulm has any relation to the similar Watchdog Garm from GARO: The Carved Seal of Flames.