He is voiced by Mitsuo Iwata in the Japanese version of the anime, while in the English version, he is voiced by Trevor Devall in season 1 and Justin Cook in seasons 2 and 3.
Marchosias expresses his will through the Divine Vessel Grimoire, which takes the form of a book. However, his true form was shown when he and Margery lost their rationality and started to rampage. His true form is a werewolf with a very large, ultramarine-colored, and fiery body. He has also a yellow colored eyes and powerful roar that could even pierce through the Seal to reach humans outside.
Marchosias is generally a very loud, outspoken and overbearing being. He talks wildly and often makes inappropriate comments without remorse. A running gag in the series occurs where he will say an inappropriate comment, with Margery hitting his Divine Vessel, oftentimes saying, "Shut up, Stupid Marcho". Like Margery, he enjoys a good fight and tends to laugh violently during combat, with such incidents being one of the rare times the two are on the same wavelength.
However, despite his wild mannerisms and battle-lust, Marchosias is a reliable comrade to Margery and sometimes acting as a mentor figure to Satō and Tanaka (and on rare occasions, even Margery herself), often displaying great perception and cunning, and can be calm and solemn when the occasion demands so. Interestingly it is the Crimson Lord that is the more cool-headed character in their partnership compared to his more impulsive contractor. He also calls out Yuji for getting ahead of himself during training, telling him that patience is virtue.