Mard Geer Tartaros (マルド・ギール・タルタロス Marudo Gīru Tarutarosu) was an Etherious and founder of the Dark Guild Tartaros. He was known as the Underworld King (冥王 Meiō)[1] and the Definitive Demon (絶対の悪魔, Zettai no Akuma),[2] having placed himself in charge of the guild in E.N.D.'s absence.[3]

Mard Geer is tall, lean and has the appearance of a young man. Often seen wearing a smirk, Mard Geer's dark purple hair is long and wavy, reaching his shoulders even whilst the majority is tied up in a very large ponytail. Additionally, his messy bangs all but obscure his dark, slanted eyes.[1]