Gengar is a dark purple, bipedal Pokémon with a roundish body. It has red eyes, a wide mouth that is usually curled into a sinister grin, and pointed ears. On its back are numerous spikes and it has smaller spikes on top of its head similar to tufty fur. Its arms and legs are short with three digits on both its hands and feet. It also has a stubby tail.
As Mega Gengar, a portion of its body sinks into the ground and its arms and tail lengthen. The lower portions turn a more reddish purple, and a third eye appears on its forehead. This new unblinking eye is oval, yellow, and allows it to see into other dimensions. The spikes on Mega Gengar's back are more numerous and pointed and additional spikes appear on its hands and tail. Its ears are now considerably longer and tilt backward. Mega Gengar has the ability to access other dimensions and has a vicious nature. It may even curse its own Trainer.
Gengar has the ability to hide perfectly in the shadow of any object, granting it exceptional stealth. However, Gengar's body acts as a heat sink. Its presence cools the temperature of the surrounding area by nearly 10°F (5°C), because it absorbs the warmth. The anime has shown that Gengar can fly and possess people and other Pokémon. In the past, Night Shade was its signature move.
Gengar is very mischievous, and at times, malicious. It enjoys playing practical jokes and casting curses, such as pretending to be one's shadow, then behaving erratically. When the quarry notices, the Gengar takes delight in its victim's terror. However, Gengar has been known to be loyal to a Trainer who treats it well. Gengar lives in caves and dark places where shadows form. It is especially fond of urban areas such as cities and back alleys, but only during the night.
A group of Gengar appeared in Illusion Confusion!, along with a group of Haunter. They were causing trouble for Ash and his friends while they were wandering through a deep forest, by creating a number of illusions that impeded their progress.
A Gengar appeared in PK15. It was one of the residents of a spooky house.
A Gengar appeared in Scary Hospitality!, under the ownership of Lon. It was used alongside Lon's Gastly and Haunter to scare visitors of the "Scary House". This included Ash, his friends, and later Team Rocket.
A Shiny Gengar that can Mega Evolve appeared in Volcanion and the Mechanical Marvel, under the ownership of Alva. It was his main battling Pokémon. However, it eventually abandoned Alva immediately after the destruction of the Mega Wave crystal that controlled it.
Gengar debuted in the opening scene of the first episode, Pokémon - I Choose You!, where it and a Nidorino battled in a colosseum on TV as a reference to the original Pokémon Red and Green games. Thus, Gengar and Nidorino were the very first Pokémon to be seen in the anime, not counting the intro.
Gengar first appeared in Holy Moltres, under the ownership of a Team Rocket Grunt. It was used to battle Blaine's Arcanine and Rapidash, and almost attacked Red.
In Pokémon Adventures, Agatha has two Gengar on her team, and many in her Ghost-type army. Her first one and her Ghost army first appeared in Lapras Lazily. One of them was used to attack Blue and Koga from the shadows, draining the latter's life force as it possessed his shadow. Eventually, it was taken down when Koga used the tail that the old woman's Arbok bit off his own, distracting it long enough for Blue's Porygon to deal the finishing blow.
Gengar is a playable character in the arcade fighting game. Moves it can use include Shadow Ball, Hypnosis, Shadow Punch, and Curse. Upon entering Burst form, it Mega Evolves and can use the Burst Attack Shadow Drop.
Pokémon Pinball: A Gengar appears in a bonus stage accessible from the Red Table. Once the player has knocked out the three Gastly and the pair of Haunter, a Gengar then appears and must be defeated to win.
Pokémon Mystery Dungeon: Red Rescue Team and Blue Rescue Team: Gengar is the leader of Team Meanies. While often accompanied by Ekans and Medicham, Gengar will often act alone and spends much of the game causing trouble, even convincing the village to exile the hero, blaming him or her for the natural disasters. However, it is later revealed that he is the cursed human. After Rayquaza destroys the star heading for the world, Gengar will post a request, asking to visit Ninetales in the hopes of bringing back Gardevoir. While she is rescued, she has no recollection of her times with her trainer, but Gengar is still satisfied to see her safe and leaves.
Pokémon Super Mystery Dungeon: Gengar appears as a recruitable character and asks the player to deliver a letter to Banette. As he is connected to Team Charm's Gardevoir, it is implied that he is the same aforementioned Gengar from Red and Blue Rescue Team, and that Team Charm's Gardevoir is the same one
When viewing Mega Gengar's Pokédex entry, the camera's view can only be moved sideways.
Gengar could be seen as a counterpart to Alakazam. They are both the last stage of a three-part evolutionary family, require trading to reach their final stage of evolution, and are capable of Mega Evolution. They are the same height, and both get shorter upon Mega Evolution. They share a base friendship of 70 and, since Generation VI, a base stat total of 500. Both are weak to the other's STAB, and they share weaknesses to the Ghost and Dark types. In the anime episode The Ancient Puzzle of Pokémopolis, a Gengar and an Alakazam are put against each other.
In the core series games, Mega Gengar has small feet that can be seen by recalling it and sending it out. In Pokkén Tournament, however, Mega Gengar does not have feet. Instead, a portal appears beneath it whenever it is airborne. Its tail will disappear as well.
Gengar and Gangar are possibly a shortening of doppelgänger, a double of a person, which is fitting for a Pokémon with a habit of pretending to be a person's shadow. The kanji 幻 maboroshi can also be read as gen and is used in words meaning phantom or illusion. There is also a striking pronunciation similarity to the Danish word genganger, a term for ghosts found in Scandinavian folklore.