As revealed in SS016, Gengar was originally caught by a Trainer living in Vermilion City, three years before the events of the new series. However, as he started losing battles and failing to catch Pokémon in Gengar's presence, he attributed his misfortunes to a curse caused by Gengar and decided to get rid of it. Before he abandoned it, he instructed Gengar to wait for him at the place that would later become the Sakuragi Institute.
As a wild Pokémon
Not knowing its Trainer had no intention of returning for it, Gengar obediently waited for its Trainer, even staying at the building as it was being converted into Professor Sakuragi's research center. It tried to scare the Professor off by haunting the building, but as he only found its actions amusing, it eventually gave up trying to frighten him.
Gengar debuted in SS011, where it resumed its mischievous activity by scaring Renji, making its presence known for Ash and Go in the process. The latter tried to catch it, but Gengar simply deflected all of the Poké Balls he threw at it. It was eventually forced to flee when Koharu commanded Yamper to use Spark. It was later seen at the end of the episode, perched atop the Institute.
Gengar briefly reappeared at the end of SS014, where it phased through the wall and watched Ash, Go, and their Pokémon after they returned from Unova.
In SS016, Gengar once again began causing trouble at the Institute and subsequently hid in Ash's shadow. When Ash ran into Gengar's former Trainer, it heard him bragging about how much he enjoyed being free from its curse. Angered by the revelation that its Trainer abandoned it, Gengar started attacking him. When the Trainer fled in fear, Gengar ran off as well. Team Rocket found Gengar and asked it to join them, but when it refused, they instead tried to capture it. Ash showed up, managed to free Gengar, and convinced it to fight Team Rocket alongside him. After Gengar defeated Team Rocket's two Dusclops, Ash collapsed from a fever he had unknowingly caught earlier. When Ash woke up at the Institute, Gengar appeared and dropped a Poké Ball for him, indicating it wanted to join his team. Ash accepted the request and caught Gengar.
With Ash
In SS018, Ash used Gengar in his battle against Visquez so he could increase his Pokémon World Championships ranking. Gengar substituted for Pikachu while going up against Visquez's Raichu and managed to defeat him with Psychic. Gengar then went up against Visquez's second Pokémon, Electrode, but was soon defeated, though not before dealing some damage on its opponent.
Personality and characteristics
Gengar and Ash
Gengar in Ash's shadow
Gengar was once loyal to its previous Trainer, obeying his instructions to remain at the future site of the Sakuragi Institute. Over time, however, Gengar became resentful of its old Trainer, even to the point of attacking him upon seeing him again and only calming down once he ran away. It grew bitter and developed a hostile nature against all humans, repeatedly attacking them with its powers. After it bonded with Ash and was caught, it became friendlier, though it still seems to have a mischievous streak, such as when it showed its concern for Ash by dumping a pile of apples on top of him.
