Xellos introduces himself as the mystery priest, but he's actually a high-ranking mazoku. He serves under one of the five subordinates, the Greater Beast.
Like his self introduction he's a figure veiled in mystery. Many have attempted to figure out his identity and intentions, but the only response they got from him was "that's a secret".
Xellos uses negative emotions of life forms like fear, hate and depression as nourishment just like other mazoku. He extremely detests positive emotions such as love, and friendship.
Xellos is known to be the most powerful mazoku excluding the 5 subordinates. He exterminated thousands of dragons during the Koma War in the past. Due to this incident, Xellos is a feared entity among the elves and dragons.