Yapool emerges as an ancient entity from an alternate dimension, frequently likened to demonic figures such as Satan or Legion due to its intrinsic malice and destructive essence. It wields a gestalt consciousness capable of merging multiple beings into singular forms like Giant Yapool, concentrating its collective hatred and intellect into a potent physical manifestation. Its primary strategy involves forging biological weapons known as Terrible-Monsters (Chojuh), synthetic entities exceeding conventional monsters in power and adaptability. These creations arise from Yapool's advanced interdimensional science and biological engineering, often fusing Earth organisms with extraterrestrial components. Examples encompass Verokron, a coral-space monster hybrid housing internal missile systems, and Vakishim, a fanged caterpillar-inspired entity able to shatter dimensional barriers and possess corpses. Yapool deploys these monsters to invade Earth, targeting Ultraman Ace and his human allies while exploiting humanity's darker traits—such as manipulating a deranged manga artist to control the Devonian-era fish monster Garan. Yapool evades direct confrontation through cunning shapeshifting and psychological warfare. It adopts numerous disguises, including a Strange Old Man hypnotizing children, a female dentist implanting hallucinatory chips in Ace's host Seiji Hokuto, and Alien Simon—a juvenile alien form utilized to erode children's trust in Hokuto. These deceptions consistently exploit emotional and moral vulnerabilities. Despite repeated defeats, Yapool exhibits near-immortality; its consciousness persists through dispersed cells, residual energy, or dimensional retreats, guaranteeing inevitable resurrections. For instance, after Giant Yapool's destruction by Ace, its cells spawned independent Terrible-Monster attacks across Earth, while its spirit regenerated. Across the Ultra Series, Yapool resurfaces as a persistent antagonist. In *Ultraman Taro*, it returns as Remodeled Giant Yapool, creating enhanced Terrible-Monsters like Oil Drinker and Remodeled Bemstar. During the Dark Spark War in *Ultraman Ginga S*, Yapool hides within a Vakishim spark doll before possessing a human host to manipulate Ultraman Victory and Ginga, only to be destroyed by Ace's techniques. It later orchestrates a multiversal crisis in *Ultra Fight Victory* by reviving Juda Spectre, the "Space Emperor," using stolen Victorium Core energy. In *Ultraman Z*, Yapool's residual energy forms a sentient Baraba that recognizes Ultraman Ace, and in *Ultraman Decker*, it collaborates with the vengeful alien Agams, providing Terrible-Monster Aribunta and amplifying Agams's hatred before being drained of memories by the Sphere and destroyed. Conceptually, Yapool's identity fluctuates across media. Initially depicted as an interdimensional race (*Ultraman Ace*), it later consolidates into a singular entity. Its design merges crab anatomy with King Enma, the Buddhist judge of the underworld, symbolizing its role as a purveyor of suffering. The name derives from Shōzō Numa's 1956 novel *Kachikujin Yapu*. In the manga *ULTRAMAN*, an alternate version exists as Seiji Hokuto's benevolent guardian and a genius scientist who contributed to Ultraman Suit technology, contrasting sharply with the mainstream incarnation. Core traits define Yapool across all appearances: it embodies ontological evil, inflicting suffering devoid of higher goals or loyalty. This is exemplified in *Decker*, where it enables Agams's destructive plans solely for personal gratification. Its intelligence manifests in long-term scheming, such as coordinating agents during its resurrections or adapting Terrible-Monsters to counter new Ultras. Despite physical destruction, its essence endures through dimensional exile, dispersed cells, or reconstituted energy, ensuring its status as the Ultra Series' most enduring adversary.