A true prankster, and also one of the most powerful Mazoku in existence.
He is a Mazoku and the priest of Greater Beast Zelas Metallium. He originally came to join Lina's party of trouble-makers on the command of Hellmaster Phibrizzo, who ordered him to take a semi-break from his usual task (finding and destroying the pages of the Claire Bible) and instead accompany Lina and prevent her from finding the book. He initially posed as a sort of unusual priest, but it was eventually revealed that he is in fact a high-ranking member of the Mazoku. Since then he has worked both for and against Lina, depending on what he and his masters decide is best for the Mazoku.
Xellos was created by Hajime Kanzaka to be a one shot character, but due to his popularity he become one of the main character.
Most of the time, Xellos comes across as merely a playful trickster, playing mostly harmless pranks on those around him. Beware if his eyes open, though: that means he's getting serious. He seems to have some fondness for Lina and her merry crew, but he will not let that get in the way of performing his function as a Mazoku and obeying the commands of his superiors. Unfortunately, Xellos is very clever, which means that if he joins with Lina it can be hard to tell whether he's there to help or hinder them. As a result, Lina's party treats him with a fond exasperation but also a wariness. Xellos doesn't share what he knows freely. He might tell a story, but the parts he leaves out of the story are probably a lot more important than the ones he tells. Most direct questions put to him and any questions he doesn't feel like answering are met with a waggle of his finger, and his catchphrase, "That is a secret."
Slayers NEXT: When Xellos first met Lina and the gang, his job was to seek and destroy the Claire Bible
Slayers TRY: Xellos is ordered to find one of Gaav's henchman
Slayers Revolution: Xellos is ordered to help the queen Ruvinagald, putting him against Lina and the gang
Slayers Evolution R: Xellos is ordered to seek and destroy the Hellmaster's Jar
Slayers Premium is the fifth of the Slayers movie