He´s one of the most powerfull Crimsom Lords and part of the Bel Masqué trinity along with Hecate and Bel Peol.
Sydonay's name is derived from the biblical demon Asmodeus, who is depicted in Collin de Plancy's Dictionnaire Infernal with the features of a chimera-like creature. Sydonai or Sidonay are among the alternate spellings for Asmodeus.
Sydonay is a Crimson Lord that takes a form of a tall, well-built man (about 180 cm) with gray hair. He wears a black suit, a pair of leather shoes, and a sunglasses. He also often seen smoking cigarette. His flame color is muddy violet.
In the Great War, he wore a complete suit of armor.
He usually stays outside of the organization, because he wants freedom to do as he wants. His hobby is serving Denizen as a bodyguard. He appreciates human civilization enough to enjoy cigarettes. He often called this world as a game, and the one he's really care is just Hecate, like he showed in the middle of a battle with Margery.
In the past, he once used the name Chi You (蚩尤 Shiyuu ), the name of a war deity who waged war against the Yellow Emperor but was eventually defeated and executed. Ironically, his ultimate adversary is Yu Xuan's contracted Crimson Lord, "Pennant of Offering" Di Hong ("奉の錦旆" 帝鴻 "Hou no Kinpai" Teikou ), which is the other name of Yellow Emperor, himself. Di Hong's divine vessel is also called "Kun Wu" (昆吾 Kongo), named after Yellow Emperor's sword, Kun Wu, which, according to the legend, could cut through jade like paper and was used to kill Chi You. It is hinted that Di Hong and Chi You (Sydonay) have fought with each other in ancient times. But in the end, contradictory to the factual legend, in the battle at Shanghai's Outlaw branch, Sydonay personally executed Yu Xuan and drove Di Hong back to the Crimson Realm.
He's voiced by Kenta Miyake in Japanese and Eli Gabay in English.