Mercurius

Karl Kraft (メルクリウス / カール・クラフト Merukuriusu / Kāru Kurafuto) was the ominous magician and alchemist who created Ewigkeit and established the Obsidian Round Table together with Reinhard Heydrich. He was the one responsible for turning the Longinus Dreidzehn Orden from a clique of elite army officers merely dabbling in the occult into a den of genuine "demons". Not only is Mercurius the one sworn friend of the Table's leader, he also stands as the only being capable of matching his sublime prowess.

Although his position could be considered that of a mentor and a father, the rest of the LDO members - with the exception of their master - shun him as a hateful taboo. His whereabouts have remained a mystery for the past six decades.

There is no other explanation at this point on why, however all we know was all characters (everybody including the main protagonist and his enemies, the LDO) in the VN and anime hated Mercurius, with the exception of Heydrich.