Milleous is the former Emperor of the the Incurseans.
Due to high rank, Milleous is referred to by his full title: "Lord Emperor Milleous, light of the Incursean Empire, destroyer of galaxies, keeper of the Conquest Ray. All living creatures tremble…."(at or before this point, the speaker is always interrupted implying a much larger title). He always gloated his daughter Attea assuring her she'll dethrone him one day and when she finally does dethrone him, his feelings are mixed between anger and pride.
Milleous first appears in X=Ben + 2, where he seeks help from Ben's Team in rescuing his daughter from Sevenseven, threatening to destroy Earth with the Incursean Conquest Ray if they refused. Eventually, Sevenseven agrees to return Attea the Attea for ransom. But once the ransom was paid, Attea paid Sevenseven twice to help her dethrone her father; but in the same episode, Milleous reclaims his throne and imprisons Attea.
Milleous briefly appears at the end of Simian Says, where he is seen purchasing the DNA Repair Guns from Simian.
Milleous reappears in So Long And Thanks For the Smoothies, where he hired Argit to retrieve the Anhilaarg from an abandoned Contemelia spaceship. However, Octagon Vreedle and Rhomboid Vreedle also came after the Anhilaarg and forced Argit to hand it over when they found him locked up in a closet by Ben. Both parties start fighting over the Anhilaarg and accidentally activate it, causing the universe as we know it to vanish from existence until Ben (inside Alien X) recreated it. However, everyone else remained unaware that the Anhilaarg even activated in the first place and believed it to be a dud.
In Tummy Trouble, Milleous appeared at the end of the episode where his daughter lied and told him that the mission in capturing the Gourmand Princess and destroying Peptos IV was a success.
In Vilgax Must Croak, Milleous orders Attea (and her recruitment consisting of Sevenseven, Sixsix and Eighteight) to kill Vilgax while he's being escorted into Incarcecon by the Plumbers while he invades Vilgaxia (Vilgax's home planet). However, the mission proved to be a failure as the Chemiren Sui Generis citizens were able to fight back, forcing Milleous to recall his fleet and leave.
In The Frogs of War: Part 2, Milleous is forcibly dethroned by his daughter and taken into Plumber custody alongside Dr. Psychobos, much to his chagrin.
In Catfight, Milleous is seen watching the battle between Attea and Bulfrag.