He 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 send his daughter to the dungeon where her mother is. He appears to not care for life as he was willing to destroy Earth with all its people; he destroyed Pluto as an example of what he was going to do to Earth if his daughter was not returned safely, and was more than wiling to imprison his daughter after taking control back.
Due to his high rank, some Incurseans and their allies call him 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).
He has a complex relationship with his daughter. While he did imprison her for attempting to steal his throne, he still praised her for her for it and said she very well may succeed one day. After she does dethrone him he is pulled between anger and pride.
In Tummy Trouble, he 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, he attempted to overthrow Vilgaxia, but due to Attea not killing Vilgax, Milleous wasn't able to invade that world and was forced to retreat.
In The Frogs Of War: Part 2, he 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.