Yapool is an evil entity from another dimension whose resentment for the Ultras has made him "immortal" and the primary antagonist of Ultraman Ace, where he bears many similarities to Satan and Legion. Yapool later returns as a major antagonist in Ultraman Mebius, serving as one of the Four Heavenly Kings under Alien Empera.
Yapool began seeking to conquer Earth with his army of Choju. Noticing Yapool's ambitions, the Five Ultra Brothers sent Ultraman Ace to Earth. Ace teamed up with Earth's defense team at the time TAC, and began working with them to thwart Yapool's plans
After the failure of his Chojus, Yapool attempted to take matters into their own hands. They sent an agent of theirs in the guise of an old man to kidnap children and send them to their dimension.
After noticing Yapool's plan, TAC decided to attack Yapool in their own dimension. They tried send Hokuto to Yapool's domain with a dimensional travelling device, but Yapool attempted to disrupt the device and send Hokuto to a different dimension. However, Zoffy showed up and allowed Minami to transform into Ultraman Ace with him. The Yapool then merged into a single entity, Giant Yapool.
Ace and Giant Yapool battled until Yapool was left unable to continue fighting. Ace then returned to Earth and freed to captured children. While Yapool recovered from the injuries he sustained, other aliens would attack Earth.
In order to get revenge on Hokuto (who had become the sole host of Ultraman Ace due to Minami's departure) Yapool took up the guise of a female dentist and implanted a hallucinogenic chip in Hokuto's teeth, causing him to see Verokron II everywhere.
After Hokuto's hallucinations led to him getting fired from TAC, Yapool unleashed the real Verokron II on a city. Yapool was defeated when Hokuto fired a gun at him that reduced him to bubbles, while Verokron II was destroyed by Ace soon after.
However, Yapool was not done just yet. The evil entity disguised himself as an Alien Simon as part of a plan to gain TAC's trust. Yapool had his strongest Choju, Jumbo King, attack a city so he could escort some kids away.
Yapool eventually revealed to Hokuto via telepathy his true identity and intentions to get revenge on TAC. Yapool was killed again by Hokuto, before he transformed into Ace and defeated Jumbo King.
After several months of laying low, Yapool made another attempt to attack the Earth. He revived Bemstar as Reconstructed Bemstar with modifications to make him stronger. Both ZAT and Taro proved unable to destroy the monster. When a man began stabbing Bemstar in its eyes repeatedly, Yapool revived Sabotender to kill the man. Eventually, Kotaro returned and transformed into Ultraman Taro to fight both monsters. Yapool revived Verokron II to defeat him, but all three of Yapool's monsters were defeated. Yapool then attempted to retreat but his ship was destroyed by Taro via the Storium Ray.
Yapool returned in episode 24 as a member of the Four Heavenly Kings and major antagonist on the show.
Yapool sent several of his Choju to Earth to menace both Ultraman Mebius and GUYS. He later met up with his fellow Heavenly Kings (Alien Mefilas, Grozam and Deathrem) to contemplate the destruction of the Ultras. Yapool announced to his fellow Kings that he would be first and only to confront Mebius. However, Deathrem wanted to be the one to fight Mebius and attempted to follow Yapool, but he was stopped by Mefilas, who told him they should wait to see what Yapool does first.
Yapool summoned Ace Killer to Earth to battle Mebius. Ace Killer easily had the upper hand in the fight, and things only got worse for Mebius when Yapool had Gadiba merge into Ace Killer to transform him into Mebius Killer. Mebius Killer brutally beat down Mebius. However, some encouragement from Aya gave Mebius the courage he needed to overpower and defeat Mebius Killer, destroying the robot with the Mebium Dynamite move.
Shortly after Mebius Killer's destruction, Yapool sucked Mirai, Aya and a smug reporter into a dark abyss containing a ruined Tokyo. Yapool then appeared before the three, and offered to let the reporter go free if he shot and killed Mirai with a large laser gun he presented to the reporter. The reporter attempted to shoot Mirai, but Mirai was able to protect himself and Aya. After firing at him many times, the report ran out of ammo and was cast aside by Yapool. Mirai then challenged Yapool to fight. Yapool assumed his giant form, while Mirai transformed into Ultraman Mebius. The two fought, with Yapool easily gaining the advantage, as Mebius was still recovering from his encounter with Mebius Killer, and could barely fight back.
Aya attempt to deliver some more words of encouragement to Mebius, but Yapool blasted her away. However, Seiji Hokuto, Ultraman Ace's human host, spoke to Mebius through his mind, telling him that if he has someone that is precious to him, he can and will overcome any foe. With that, Mebius' power and spirit were restored, allowing him to stand up and assume his Burning Brave Mode. Mebius then attacked Yapool and managed to defeat him with a Mebium Bust. Not dead yet, Yapool told Mebius that even stronger foes than him would come before collapsing to the ground and exploding into dust. Yapool was finally finished.
Grozam later informed Mefilas that Yapool was killed. Not caring, Mefilas informed Grozam that Deathrem had already set off for Earth.
Ultraman Mebius Gaiden: Ghost Reverse
Yapool as later revived along with the other three Heavenly Kings and possessed the body of his creation, Mebius Killer. The Four Heavenly Kings then attacked the Ultra Brothers to bring together the Giga Battle Nizer and Mechazam so they could revive Alien Empera. Their plans are ultimately foiled when Ultraman Mebius gets rid of the Giga Battle Nizer and Mechazam sacrifices himself to prevent the Empera from returning. Yapool was later destroyed by Ultraman Ace with a shot from his Metallium Ray.
Ultraman Ginga S
Angry at Android One Zero for using Vakishim's Spark Doll without permission, Yapool cast her out and took control of the Choju, sending it back to his home dimension. Yapool then told One Zero and Exceller that he would deal with Ginga before disappearing.
Yapool then possessed Gouki Matsumoto and began rampaging. However, Gouki began struggling with Yapool for control after seeing a pregnant woman needing help, prompting the Yapool to turn into his giant form. Ginga later kicked Yapool, freeing Gouki from his possession. He then had Vakishim to retreat back into their dimension but Ultraman Victory managed to bring it back. Yapool decided to battle Ginga himself but after Vakishim's defeat, Yapool attempted to return to his dimension. However, Victory stopped him, allowing Ginga to destroy him using the Metallium Ray.
Ultra Fight Victory
Yapool later returned with the intention of using the Victorium Core to revive Juda. He stole Sakuya's Victorium Necklace and used it to siphon energy from the Core itself. Yapool then revealed himself to Ultraman Victory and sent Doragory, Verokron and Vakishim to fight Leo and Astra. He also sent Ace Killer to keep Ace from saving Ginga., copying Ultraman Victory's UITrans and giving it to the robot, declaring him Victory Killer.
Yapool summoned Lunaticks to kill Ginga, but both it and Victory Killer were destroyed by Victory and Shepardon. Yapool then traveled to Planet Guar, where he used his Victorium to commence Juda's revival. Ace, Victory and Ginga soon arrived, prompting Yapool to engage them in battle. Yapool initially overpowered the Ultras, until they merged into Ultraman Ginga Victory. The fight then swung in the Ultras' favor, who defeated Yapool with an Ultra Fusion Shoot. Before dying, Yapool declared that none could stop Juda's revival. As he exploded, Yapool sent his energy to the Space Emperor, reviving him.