Gengar is a dark purple, bipedal Pokémon with a roundish body. It has red eyes and a wide mouth that is usually curled into a sinister grin. Multiple spikes cover its back, and it has large pointed ears. 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. The lower portions turn a more reddish purple, and a third eye appears on its forehead. This new unblinking eye is oval and yellow and allows it to see into other dimensions. The spikes on Mega Gengar's back are more numerous and pointed. Additionally, spikes appear on its hands and tail. Its ears are now considerably longer and project backward.
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, because it absorbs the warmth. In addition, Gengar possesses the ability to fly through the air, as demonstrated in the anime.
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 shadows of rooms, 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.