Upon its debut, Gengar was rather violent towards humans and other Pokémon. However, it is later revealed that its rogue nature was due to being abandoned by its original trainer, for whom Gengar had waited three years to return. After it started bonding with Ash, Gengar became a much friendlier Pokémon and gained complete trust in him.
For the majority of time, Gengar is a prankster, but is still friendly and grateful towards those who are kind to it, especially Ash.
Gengar was left by its original Trainer, who was told to wait for him until he come back. Since then, Gengar waited three years for its trainer to return, but he never did. When it became clear that its trainer was never coming back upon witnessing that it was only a woman an a housing agent that was looking around the building, Gengar grew bitter and started haunting the institute after he scared off both the agent and the woman.
Gengar first appeared at the Sakuragi Institute, where it maliciously attacked Renji and incapacitated Françoise, who were attempting to work in the Institute's basement. It then made an appearance in front of Professor Sakuragi and Kikuna (utterly terrifying them for a brief moment) before vanishing again. Ash and Go later encountered it in the main corridor, futilely attempting to capture and battle it before it took off again. When Koharu entered the Institute after a long day at school, Gengar went to attack her, being stopped by the efforts of Ash and Pikachu. Professor Sakuragi then told his daughter to command her family's Yamper to fight the evil Pokémon and send it away. Since Koharu didn't like Pokémon and wasn't a battler, she had a dangerously long internal debate for a while. However, she eventually gave in and ordered Yamper to use Spark, resulting in the malicious Pokémon being blasted out of the building. While everyone was saved for the moment, their troubles were not over during the nighttime, Gengar stood atop the Sakuragi Institute, apparently plotting its next scheme.
It later reappeared when it was watching Ash and Go as it began to set up its new tricks.Sometime later, Gengar returned to cause more trouble at the institute. It continuously scared Renji, made messes everywhere, turned the lights on and off and made the doors open and close by themselves. It later hid in Ash's shadow and secretly took control of him, making him do various weird things. Ash soon ran out of the institute and began strolling down the streets of Vermillion City. But during his walk, he ran into two people. One of them was a boy that Gengar happened to know prior to its stay at the institute. The boy turned out to be its original trainer, who told Ash and Go that he ordered it to stay at the institute and made it wait for him until he returned. As Gengar listened, it heard its former trainer bragging about getting rid of it and had no intention of returning for it. Gengar became enraged and attacked its former trainer and his girlfriend in an act of revenge.
Gengar ran off to cool itself down, but Team Rocket appeared and captured it. Luckily, Ash and Pikachu freed Gengar and sent them blasting off. Ash then asked Gengar to join him, but collapsed in its arms before Gengar could respond. After Ash woke up from a fever, Gengar gave Ash some food and an empty Poké Ball, showing that it had accepted his offer and allowed itself to join his team.
Ash used Gengar in his first battle of the Pokémon World Championships against Visquez as a substitute for Ash's Pikachu. It was victorious against Raichu but soon lost to Electrode, not before managing to do some damage to it with Psychic.
- Yasuhiro Mamiya (Japanese)
- Gengar is Ash's first:
- With Gengar's capture, Ash now officially owns or has owned at least one Pokémon of all 18 types.
- Just like Ash's Charizard as a Charmander, Gengar was told by its trainer to stay and wait for his return, but turns out that the trainer abandoned it out of selfishness. It's also similar to Ash's Squirtle, Infernape (as Chimchar) & Pignite (as Tepig) who were also abandoned by selfish trainers who were glad to be rid of them before Ash took them in.
- Gengar also has some things in common with Ash's Muk. Both are purple Generation 1 poison-types that were already fully evolved when Ash caught them near Vermillion City.
- Prior to being captured by Ash, Gengar had noticeable wrinkles under its eyes. After joining Ash's team, the wrinkles disappear, symbolizing how it has a new life and has become a much happier Pokémon.
- In many ways, Ash catching Gengar is a way to make up for the Haunter he tried to use against Sabrina since he never officially caught it.
At Oak's Lab
At Kukui's house