Ken Utonium is Professor Utonium's 8-year-old son and a boy genius in Demashita! Powerpuff Girls Z. He is somewhat responsible for turning three regular girls into the Powerpuff Girls Z and numerous other characters into villains using Chemical Z. He used it to blast a glacier in order to set the weather back to normal, but the impact resulted in the explosion of several lights, which affected all those who came in contact with it thus, black and white lights break apart hitting various people, animals or items (even his robotic dog Poochi). Despite being younger than the girls, he acts a lot more mature and the education he receives from his father is considered more advanced than what the girls learn in their school, earning his PhD at an early age. Ken, in a later episode, attended school in order to gain social skills and make friends. While he considers grade school life boring, he has made several friends, including Jou (who was originally his rival) and Kuriko (Momoko's little sister).
Ken sees the girls as older sisters and has to often put up with them, though he still cares for them deeply. Ken interchanges between calling Professor Utonium Papa and Professor. During a serious situation (such as a monster attack or investigative purposes) he will try to refer to Utonium as "Professor," but in less serious situations (like packing a lunch) he will call him "Papa." Ken often corrects himself, because he usually uses the wrong honorific (e.g. "Papa, I mean, Professor").