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Gastly has no true form, due to its body being 95% poisonous gas. However, it consistently appears as a black, spherical Pokémon surrounded by a purple haze. It has a wide, pink mouth with two visible fangs. Though its eyes seem to extend past its round body, visible eyelids surround them. The toxic gas surrounding the main body can induce fainting and suffocation, and is capable of enshrouding an Indian elephant within two seconds. Because it is mostly gas, Gastly will dwindle away when exposed to strong winds.
The anime suggests that Gastly has a playful personality, often frightening and playing pranks on people for fun. However, it has shown a willingness to preserve or pass on the memories of departed humans. It has further been shown changing form and creating illusions, and it is capable of phasing through solid objects and forming tangible hands out of its gasses. Gastly is often seen in old, dilapidated buildings, but can also be found in caves.