Xellos tends to behave like a playful trickster, answering nearly all questions directed toward him with "That is a secret." He seems to delight in the frustration that ensues with his withholding of information. In 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. 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.
Xellos is widely considered to be the most powerful Mazoku under the five lords. As such he is nearly immune to all Black Magic. 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.