Upon its debut, Gengar was rather hostile 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, though it still retained its mischievous streak.
For the majority of time, Gengar is a prankster, but is still friendly and grateful towards those who are kind to it, especially Ash. Similar to Ash's Palpitoad, Gengar shares Ash's strong love of battling and follows his commands without question. In combat, Gengar makes full use of its Ghost typing, such as being able to phase through solid objects. However, it suffers against faster opponents, like Electrode and Lucario. It can also disappear and reappear at will, hide in the shadows, and is able to decrease the temperature of its surroundings.
Gengar also hates being wrongly blamed for things, as it hits Ash with a Shadow Ball for thinking it was behind the Pokémon food shortage at the Cerise Laboratory.
Prior to the start of the Journeys series, Gengar originally belonged to a trainer in Vermillion City. However, the trainer began to experience a string of bad luck, such as losing battles and not being able to catch Pokemon. Believing that Gengar had cursed him, he decided to get rid of it by telling it wait for him until he came 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 and a housing agent that was looking around the building, Gengar grew bitter and started haunting what would later become the Cerise Laboratory after he scared off both the agent and the woman.
Gengar first appeared at the Cerise Laboratory, where it maliciously attacked Ren and incapacitated Francois, who were attempting to work in the institute's basement. It then made an appearance in front of Professor Cerise and Chrysa (utterly terrifying them for a brief moment) before vanishing again. Ash and Goh later encountered it in the main corridor, futilely attempting to capture and battle it before it took off again. When Chloe entered the institute after a long day at school, Gengar went to attack her, being stopped by the efforts of Ash and Pikachu. Professor Cerise then told his daughter to command her family's Yamper to fight the evil Pokémon and send it away. Since Chloe 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 Cerise Laboratory, 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 Ren, 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 Vermilion 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 Goh 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 World Coronation Series 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.
When Ash and Goh were discussing who was behind the disappearance of the Pokémon food at Cerise Laboratory, Ash suspected Gengar of being the culprit, only for Gengar to appear and hit Ash with a Shadow Ball for the accusation, much to Pikachu's anger.
Gengar was later used in Ash's battle in the World Coronation Series battle tournament in the Kalos region. Gengar went up against Korrina's Mienshao, but Mienshao switched out with its U-turn attack and Lucario proceeded to defeat Gengar.
- 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 and Squirtle are also Generation 1 Pokémon from the Kanto region that caused mischief until they met Ash, had a change of heart upon seeing that he cared about them, and willingly joined his team. Thus they became formidable battlers under Ash's training. They also bonded with Pikachu.
- Gengar also has some things in common with Ash's Muk. Both are purple Generation I poison-types that were already fully evolved when Ash caught them near Vermilion City. They both caused trouble prior to their capture, and become very affectionate afterwards.
- 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.
- Gengar is the first Pokémon in Journeys to know 4 moves upon capture.
- In comparison, both Dragonite and Riolu started out with only 1.
At Oak's Lab
At Kukui's house
At Cerise Laboratory