Xellos is known as the "trickster priest" or "mysterious priest." After introducing himself to the group, Xellos comes and goes as he pleases often appearing just to give Lina information. Sometimes he gets in their way and sometimes he helps them out in a desparate situation. However, Xellos has his own reasons for following Lina, but that's a secret. Normally, Xellos wears a happy smile and his eyes are closed, but when his eyes glint open, be prepared for trouble.
Xellos is no ordinary mazoku. He is both the general and priest of Greater Beast Zelas-Metallium, one of the five demon lords under Ruby-Eyed Shabranigdo. Most mazoku lords have two primary subordinates: a general and a priest. However, Zelas-Metallium chose to have only one servant making Xellos both her general and priest. This makes Xellos the single most powerful mazoku that is not a lord.
Xellos has almost no weaknesses. He cannot really be hurt in combat because he can regenerate fairly quickly. Also, Xellos is resistant to magic and can shrug off attacks by other Mazoku because they are simply not strong enough to hurt him.
Xellos is responsible for the deaths of hundreds of dragons during the War of the Monsters' Fall. This is why Filia hates Xellos with a passion. He even earns the nickname "raw garbage" from her. Despite their hatred for one another, Xellos and Filia do make a great couple.
