Ash's 8th Pokemon. Ash hoped Haunter would be able to beat Sabrina's Kadabra, but Haunter served a different role in the battle, but was still crucial to Ash's victory. After the battle Ash left Haunter with Sabrina so it could continue to make her happy.
After being defeated soundly by the psychic gym leader Sabrina, Ash and his friends began searching for a ghost type pokemon. The search took Ash to a supposedly haunted tower where he found a trio of ghosts, a Ghastly, a Haunter, and a Gengar. The ghosts were only looking to play jokes and have fun which led to Ash and his pikachu being killed from a loose chandelier. The ghost's took Ash and pikachu's disembodied souls around the tower but soon Ash noticed how sad his friends were and needed to return to the living and continue on his journey to be a pokemon master. After returning to his friends and about to set off again on his journey Ash is surprised when the Haunter decides he wants to join Ash as his own pokemon.
Haunter joined Ash, but was hardly obedient, Haunter disappeared in the fight with Sabrina's Kadabra, but soon appeared, and began trying to make Sabrina laugh, eventually it succeeded, and with the psychic link between Sabrina and Kadabra, Kadabra also began laughing, allowing Ash and Pikachu to easily defeat Kadabra, after the battle, Ash realized how positive Haunter was for Sabrina, and decided it was best for Haunter to remain at her side.