Haunter

Haunter is the second evolutionary stage for Gastly. Because of its ability to slip through walls, it is rumored to be from another dimension. In "The Tower of Terror," Ash and Pikachu battle their way through the wacky shenanigans of three ghosts at the Lavender Town Tower, so Ash can acquire a ghost Pokemon in order to defeat Sabrina. When Ash realizes all Haunter wants is to have a little fun, they leave together as friends.

First appearance: Episode 23—"The Tower of Terror"