Xellos Metallium is a prominent character in the anime and light novel series Slayers. He is a powerful Mazoku (demon) serving under the Greater Beast Zellas Metallium. Xellos is known for his sly, mischievous personality, cryptic speech, and his catchphrase, 'That is a secret.' While acting as both an ally and antagonist throughout the series, his true motivations are often hidden, making him unpredictable. Xellos provides guidance to the protagonist Lina Inverse but always pursues his own mysterious goals.