Gastly is a dual-type Ghost/Poison Pokémon introduced in Generation I.
It evolves into Haunter starting at level 25, which evolves into Gengar when traded.
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 can be found in caves, but the anime has shown it living in old, dilapidated buildings, as well. Its gaseous body will dwindle away when exposed to strong winds, so they huddle together in decent-sized groups under house eaves to avoid the wind. It may also cause flickering lights in abandoned houses. In the past, Night Shade was its signature move.