Xellos

ゼロス

Xellos frequently introduces himself as the Trickster Priest, a title justified by his tendency to mislead or cheat to achieve his goals. (He has stated that he never outright lies on principle; his clues usually contain at least a fraction of truth.) He often plays pranks simply to observe the reactions of those around him, or manipulates others into doing his work, seemingly just because he can. His famous catchphrase is "Sore wa himitsu desu," literally "That is a secret." Despite his enormous powers, he prefers subtlety and manipulation over violence, although when Xellos does enter a battle, he is known to be a ruthless opponent.

Spoiler

In the novels, the Slayers group generally abhors his presence, while in the anime he is treated as more of a tolerable nuisance.
In the anime, Xellos and Lina seem to have a sort of mutual respect for each other. He seems to enjoy the chaos that follows Lina, and follows her for the sake of fun even if he is not ordered to do so. In the novels Xellos seems to be more distant around her, looking somewhat troubled by the fact that Lina, unlike other humans he has dealt with, is able to see the bigger picture of things and is not tricked so easily.