Xellos often introduces himself as the Trickster Priest for good reason—he will mislead or cheat to achieve his goals. He has stated that he never outright lies on principle; his clues usually contain at least a fragment of truth. Frequently, he plays pranks just to observe others' reactions or manipulates them into doing his work, seemingly just because he can. His famous catchphrase is "Sore wa himitsu desu," which means "That is a secret." Despite his immense powers, Xellos prefers subtlety and manipulation over violence. However, when he does engage in battle, he is known to be a ruthless opponent.
Xellos was created by Zelas during the Kōma War, approximately 1013 years before Slayers TRY. Unlike other subordinates of mazoku lords, Xellos is the sole immediate servant of Zelas Metallium, making him exceptionally powerful due to a significant portion of her power being invested in his creation. Consequently, Xellos is the most formidable among all priests and generals serving Ruby Eye Shabranigdu's subordinates.
During the Kōma War, Xellos singlehandedly slaughtered thousands of golden dragons, earning their fear and hatred ever since. They refer to him as the Dragon Slayer, although he dislikes this name, likely because it contradicts the image he wants others to perceive. After the war, Xellos's duty for a thousand years was to locate and destroy stray copies of the Claire Bible to prevent humans from easily defeating mazoku with Zanaffar or utilizing the power of the Lord of Nightmares. This was not what he told Lina and her friends in Slayers NEXT, where he claimed to be a priest seeking a missing book.
Xellos is widely regarded as the most powerful Mazoku under the five lords. Consequently, he is nearly immune to all Black Magic, except for spells invoking Shabranigdo or the Lords. However, he remains vulnerable to spells powered by the Lord of Nightmares, though his speed makes it unlikely for anyone to successfully cast such spells against him.
In the novels, the Slayers group generally despises his presence, while in the anime, he is seen more as an annoying but tolerable figure. In the anime, Xellos and Lina seem to share a mutual respect. He appears to enjoy the chaos surrounding Lina and follows her for amusement, even without orders. In the novels, Xellos seems more distant around her, somewhat troubled by Lina's ability to see the bigger picture and resist his tricks more easily than others.
