Deep within the walls of Caislean Merlin hides Diabal, a horned individual who uses a magical device to project large, lifelike illusions called Spectrums, which Magic Knights often mistake for actual Nemesis.

He is a young man with long brown hair and small horns. His eyes are white from blindness, and his body bears the traces of past torture, hidden by bandages covering his face, hands, and arms. On his left hand, two fingers are cut halfway, and on his right, the middle finger is half-cut.

Once naive and carefree, Diabal is now profoundly paranoid after a betrayal by someone he trusted, forcing him into a life of hiding and wariness. This betrayal came from Piodon, who originally freed him from the Inquisition. Piodon raised and trained him in the ways of Fantasia, but later delivered him back to the Inquisition when he asked too many questions. After enduring torture, Diabal escaped and has been hiding from Piodon ever since.

As one of the infected, he can resist Nemesis and wield Fantasia with his bare hands. He shares a unique ability with other horned infected: a physical transformation that covers his body in black, gangrene-like marks, multiplies his strength tenfold, and allows him to generate violent Fantasia waves. He is also awakened to the shamanic vision, which compensates for his blindness by sensing the pulsations of Fantasia, though he remains unaware of this ability and cannot enter the shamanic plane at will.

A younger version of Diabal, from when he first met Piodon, is depicted on the back cover of Volume 8.