Prior to the start of the manga, Gengar was used to fight Professor Oak in the finals of the Indigo League tournament. Despite battling against his Kanga for hours it eventually lost. Afterwards, it was used to battle Oak again to get answers of why he left Agatha's research group.
Gengar first appeared in Lapras Lazily, where Agatha sent it out against Blue and Evan, who had arrived at the abandoned Power Plant where she was hiding in search of Ethan's lost Haunter. Even though its initial Night Shade was blocked by Blue's Golduck's Hydro Pump, it then used Hypnosis to put Blue to sleep and trap him in a coffin of mist, where he was then tortured by Gengar's Dream Eater and Confuse Ray. However, Blue was able to break free from Agatha's spell, and use Golduck and his Pokédex to tell Ethan to distract Gengar, so that the Dream Eater would weaken, allowing him to escape from it with his Scyther. Although Agatha claimed that his Scyther wouldn't be able to hurt her formless Ghost-type Pokémon, Blue was able to recall the lessons he had learned from his teacher, which allowed Scyther to defeat Gengar with a Slash attack.
Agatha also used her Gengar in Muk Raking, during the battle against Blue and Koga at Cerise Island. Gengar managed to paralyze Blue with Lick, making him unable to see anything. Although Blue's paralysis was later cured by Koga's Parlyz Heal and they were able to defeat Agatha's Arbok and seemingly knock out Agatha as well, she secretly sent her Gengar to follow them, hiding in their shadows and slowly draining their life energy. As there was no Trainer to give command to it, Blue wondered why Gengar kept attacking them. Eventually he figured out that it was actually reacting to even the slightest of sounds they made, attacking whatever caused the sound. Koga figured out a trick on how to defeat Gengar, and, by using the smoke of his Koffing, sent a message of his idea to Blue: since his Arbok's tail had earlier been cut off while battling against Agatha's Arbok, Koga used a secret ninja trick to make his Arbok's severed off tail move, causing Gengar to react the sound and come out of the shadows, giving Blue the chance to attack. The plan worked, allowing Blue's Porygon to defeat Gengar with a Psybeam. Afterwards, Blue used Koga's Golbat's Confuse Ray to light up the surroundings, cutting the shadows to a minimum and preventing Gengar from escaping unnoticed.
Agatha also has multiple other Gengar in her army of Ghost Pokémon, which she sent to attack Cerulean City.
Personality and characteristics
As for the most of its species, Gengar is mischievous, and likes to slowly serve the death for its victims by slowly draining away their powers. Gengar is Agatha's main battling Pokémon. It has the ability to hide in shadows.
Agatha's Gengar's only appearance was in The Scheme Team, where she used it in a battle against Ash and Pikachu at the Viridian Gym, which Agatha was temporarily taking care of. At first, Ash told Pikachu to use Quick Attack, utterly forgetting that Normal-type moves don't affect Ghost-type Pokémon. The Shadow Pokémon was also able to dodge Pikachu's Thunderbolt by turning into a shadow and sliding out of the way of the attack. Gengar then used Shadow Ball, which Pikachu dodged before hitting Gengar with Iron Tail. Gengar then attacked with another Shadow Ball, which hit Pikachu head on.
The Mouse Pokémon, however, quickly recovered from the hit, and went in for another Iron Tail. Before the attack could hit, however, Agatha had Gengar use Double Team, creating multiple Gengar clones. As Gengar and its clones prepared to use Shadow Ball again, Ash had Pikachu use Thunderbolt on all of the Gengar, breaking the Double Team and hitting the real one. Even though the Shadow Ball was still fired, Pikachu was able to dodge it and hit Gengar with another Iron Tail. Ash prepared to finish the battle with a Thunder attack, but Agatha launched her powerful Hypnosis-Dream Eater combo, defeating Pikachu.
Personality and characteristics
Very little is known about Gengar's personality as it was only seen battling Pikachu. However, it had no trouble following commands coming from Agatha during the battle. Gengar is a strong battler as it was unfazed by some of Pikachu's attacks.
Agatha's Gengar appeared briefly in The Challenger, being the last Pokémon Agatha used in her battle against Blue. It was defeated off-screen, allowing Blue to move on to Lance's room. Agatha keeps Gengar's Poké Ball embedded in her cane.
In other manga
Gengar in Pokémon Pocket Monsters
In the Pokémon Pocket Monsters manga
Gengar's first and only appearance was in Panic on the Luxury Liner?!, where it was first seen spying on Red. It was hidden after Agatha transforms from a Golbat to her human self. It reappeared after Agatha changed back into a Golbat.
In the Pokémon Zensho manga
Gengar in Pokémon Zensho
Agatha's Gengar was seen in Indigo Plateau, where she used in her battle against Satoshi. It was defeated by Satoshi's Alakazam, allowing Satoshi to advance to the next room to face Lance.
In the games
Gengar has appeared as Agatha's main Pokémon in every game she has appeared in, and, besides her Arbok, been the only Pokémon she has used in her every team.