Milleous has a daughter named Attea, who overthrew him in X = Ben + 2 for a short while, and in the same episode he recovers his throne and sends Attea to the dungeon where her mother is. He appears to not care for other life, as he was willing to destroy Earth, along with all its people; he destroyed Pluto as an example of what he was going to do to Earth if Attea was not returned safely, and was more than wiling to imprison Attea after taking control back.
Due to his high rank, some Incurseans and their allies call Milleous by his full title: Lord Emperor Milleous, light of the Incursean empire, destroyer of galaxies, keeper of the conquest ray, all beings tremble- (at or before this point, the speaker is always cut off, implying a much longer title).
Milleous has a complex relationship with Attea. While he did imprison her for attempting to steal his throne, he still praised her for it and said that she may succeed one day. After she does dethrone him, he is pulled between anger and pride.
In Simian Says, Milleous bought a DNA altering ray gun from Simian with a high price.
In Tummy Trouble, Milleous was seen at the end of the episode, where he believed that Attea destroyed Peptos XI to send a message to the entire universe. Milleous then stated that the entire galaxy will be next.
In Vilgax Must Croak, Milleous attempted to overthrow Vilgaxia, but due to Attea not killing Vilgax, he wasn't able to and was forced to retreat.
In The Frogs Of War: Part 2, Milleous was forcefully dethroned after being taken into Plumber custody. His replacement was his daughter Attea, whose plan had allegedly been to take over the Incursean armada all along.
In Catfight, he makes a small cameo while watching the fight between his daughter and Bullfrag. Despite being betrayed, he still cheered on Attea, though he comments that she could do a lot better than choosing Ben for a consort.