The ruler of the extradimensional space plotting the invasion of Earth. While possessing advanced intelligence and knowledge of interdimensional science, it is extremely cruel and insidious, sparing no means to achieve its goals. It sends forth the Terrible-Monsters, biological weapons stronger than monsters, created by the Terrible-Monster Manufacturing Machine, to enslave humanity, destroy the sun, and eliminate Ultraman Ace, who stood in its way towards Earth invasion. Additionally, Yapool subjugates numerous aliens and extradimensional beings as its subordinates to attack Earth.

Starting with the deployment of Verokron, the Missile Terrible-Monster to Fukuyama, Hiroshima Prefecture, Yapool sent numerous Terrible-Monsters and aliens to Earth, repeatedly attacking towns and assaulting the TAC base, aiming to defeat Ace, only to be thwarted at every turn by TAC and Ace.
Subsequently, when Seiji Hokuto and Yuko Minami infiltrated the extradimensional space transformed into Ace, Yapool gathered all its forces to confront them, merging into a single entity known as Giant Yapool, the Extradimensional Terrible-Being.