Marcosias is a Crimson Lord from the Crimson Realm who is bound to the Flame Haze Margery Daw. His true name is Claw and Fangs of Violation, and his flame shines in an ultramarine color. He resides within a divine vessel known as the Grimoire, which takes the form of a large book. When his true nature fully manifests in the human world, he can take the shape of a gigantic wolf.

In personality, Marcosias is loud, talkative, and fond of making wild, rude comments. He frequently teases Margery, which often earns him a retaliatory punch across the room. Beneath this bickering lies a genuine, long‑standing bond; he cares for Margery as a trusted battle companion. He is also battle‑loving and finds satisfaction in combat, and his continued involvement as a Flame Haze contractor is largely driven by his own appetite for fighting.

His primary motivation is the thrill of battle and the desire to support his contractor in their shared struggles. He and Margery have fought together since the Middle Ages, forming a partnership built on easy trust despite their constant squabbling.

In the story, Marcosias acts as the Crimson Lord contracted to Margery Daw, providing her with power through the Grimoire. He appears as a supporting character in Shakugan no Shana S and the broader series, aiding Margery in combat and offering running commentary. His most important relationship is with Margery; he interacts with other Flame Hazes and Crimson Lords but remains most closely tied to her.

Development for Marcosias is limited; his personality and role stay consistent throughout the series. His notable abilities include wielding ultramarine flames, manifesting partly or fully as a giant wolf, and residing within and controlling the Grimoire, which Margery uses as a tool in battle.