Gengar was a Pokémon, who stole people's items and hoarded them. For that, the people hated it and the Island Kahuna sealed it in a stone on Ula'ula Island. Thus, the people wrote stories about it, and gave it the name "Greedy Rapooh".
After Jessie had crushed a stone with her feet, Gengar emerged out of it to scare Team Rocket and stole their Darkinium Z. Acerola, who told the tale about the Greedy Rapooh, realized the ghost was real, and tried to run with the children. Since Greedy Rapooh intercepted them, Acerola reminded that Greedy Rapooh would be hated if it continued its misbehavior. However, Gengar snatched Acerola and carried her on its back and flew off into the sky. After stealing berries and a nugget from a Drampa and a person, and playing pranks on other Pokémon and people, it was soon approached and attacked by Mimikins, who used Shadow Claw. However, the attack didn't do damage against Greedy Rapooh, who dropped Mimikins to the ground. Acerola dived down to catch Mimikins, but fell on a tree and hurt herself. Greedy Rapooh approached Acerola, who noted it was concerned about her, and told it that she was fine. She also noticed it copied her moves, but told that she had to go home. Greedy Rapooh refused, and offered her gifts that it had collected. Since Acerola still refused, Greedy Rapooh tried to absorb Acerola, only to be interrupted by a Shadow Ball from Mimikyu. It chased after Team Rocket, who had retrieved their Z-Crystal back, but it took control of Meowth, who attempted to use Fury Swipes. Greedy Rapooh went out of Meowth's body, after it was attacked by Jessie's Mimikyu, and possessed Jessie, whom James's Mareanie hit with Knock Off. who were brought back to their senses by Mimikyu and Mareanie respectively. Acerola came with Nanu, who gave Team Rocket a Z-Ring: the group defeated Greedy Rapooh by having Mareanie use the Z-Move Black Hole Eclipse. Acerola noted that Greedy Rapooh wasn't exactly bad, since it offered her gifts, and assumed it was just lonely. Thus, she promised to play more with Greedy Rapooh, which made it happy, but Nanu swore to seal it back in the stone if it continued misbehaving.