Xellos (ゼロス, Zerosu) (sometimes spelled Xelloss) is a fictional character and a primary figure in the Slayers fantasy comedy light novel series and its manga, anime and other adaptations.

Xellos is the single most powerful servant of Zelas Metallium, who in turn is one of the five retainers of Ruby Eye Shabranigdo, one of the Slayers series' evil deity-like beings.

[2] Xellos behaves like a trickster, answering nearly all questions directed toward him with "Sore wa himitsu desu", literally "That is a secret."

For most of the second season, Xellos acts simply for comedy's sake, frequently breaking the fourth wall while subtly advancing the plot.

Generally, Xellos' mood can be determined by the state of his eyes: if they are closed, he acts as the "Trickster Priest," but when he opens them, he reveals his dark nature.

As a Mazoku, his main goal is to return the universe into the nothingness of the Sea of Chaos, but near the end of the third season, he helps Lina Inverse and the others save the world.

He is immune to most magic; nothing short of a direct hit from Lina's Ragna Blade or Gourry's Sword of Light can stun him, assuming that his enemies are fast enough to strike him in the first place.

In the novels, Xellos is more distant around her, somewhat troubled by the fact that Lina, unlike other humans he has dealt with, can see the bigger picture and is not easily manipulated.

Zelgadis holds a powerful hatred towards the Mazoku because in the second episode of NEXT, Xellos destroys a Claire Bible manuscript, which the chimera thought might have contained the cure to his state.

Perhaps due to his overpowered status, Xellos would not appear directly in any of Slayers video games for decades besides a China-only crossover collaboration event in Ragnarok M in 2021.