Xellos tends to behave like a playful trickster, answering nearly all questions directed toward him with "Sore wa himitsu desu," literally "That is a secret." He seems to delight in the frustration that ensues with his withholding of information. For the majority of the second season, Xellos acts simply for comedy's sake frequently breaking the fourth wall and addressing the viewers; all the while subtly and even sinisterly advancing the plot. Generally, Xellos' mood can be determined by the state of his eyes. If they are closed, he is acting as the "Trickster Priest," but when he opens them he brings forth the Mazoku. However, on occasion he opens his eyes out of mere surprise (as in the fourth episode of NEXT, when Lina reveals her knowledge of the Pledge Stone).
Xellos' true intentions always seem to be a mystery. 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 series he aids Lina and the others in saving the world instead. Even though he tends to manipulate Lina and the others he has saved their lives on few occasions, such as saving Martina Zoana Mel Navratilova from bandits for no apparent reason, and saving Filia during their battle with Valgaav. The reasons for doing so are never given which leads Xellos to be one of the many enigmas of the series.
Xellos is extremely loyal to Zelas Metallium and usually acts only if ordered (although at times he has aided Lina's group without having been ordered by Zelas), and it is assumed that he would gladly follow any of her requests in order to keep her pleased, even if he is not pleased with them himself (an example of this is when he was ordered to offer to kill Lina Inverse if ValGaav joined the Mazoku side in TRY).
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.
Zelgadis holds a powerful hatred towards the Mazoku because in the 2nd episode of NEXT, he had destroyed a Claire Bible manuscript which, the chimera thought, might have contained the cure to his appearance. This naturally does not bother Xellos one bit, even though he is the only one who does not tease Zelgadis about his appearance. When Amelia makes one of her heroic, "justice filled" speeches Xellos listens with amusement and mild curiosity, although they do tend to disturb him at times. When she makes endless comments about how beautiful and wondrous the world and life are Xellos becomes extremely weakened, and even cringes at her minor comments. So great is the effect of these speeches on Xellos that, when forced to fight him, the group has even used one of them as a weapon to render Xellos helpless.
Filia is the only being capable of annoying Xellos. Even though he tries to be polite to her, she continues to insult him and call him names like Raw Garbage (namagomi). Xellos likes to tease Filia by the fact that she is part of the dragon race, he makes comments like " Those of the Dragon Race would rather use violence than trying to compromise", or anything along those lines. Xellos seems to have a personal enjoyment of not only making fun of her, but he seems to like tricking her the most. Her hatred towards him comes from the fact that he killed many dragons during the Kõma Wars (War of the Monsters Fall) 1000 years ago. By the end of the series, they learn to tolerate each other and are capable of having at least one decent conversation.
Xellos is widely considered to be the most powerful Mazoku under the five lords. As such he is nearly immune to all Black Magic, only the spells that call upon Shabranigdo or the Lords can even faze him; however, he is vulnerable to spells that use the power of the Lord of Nightmares, though given his speed it is unlikely that he would allow someone to cast the spell against him.
Immensely powerful, Xellos is capable of annihilating nearly anything using his own power. This is best shown by his destruction of an entire clan of Golden Dragons. However, he tends to underestimate his opponents, leading to himself getting beaten severely by ValGaav. Like all Mazoku, Xellos can teleport anywhere he can imagine by phasing to and from the Astral Side.
"Additional information about myself: There is much more to me than just how you see me in Slayers... of course it is a SECRET!"
