Character: Ghost (Satoshi)

Haunter (Spectre in the beta versions of Red/Blue) takes the shape of an angular floating face with disembodied hands. It has a sinister look, complete with fangs and claws. However, when approached by people, it has a habit of reverting to a spherical form that obscures its eyes, mouth, and hands. Haunter is considered in the Pokédex to be a 'gas type' Pokémon, because of the overall structure of his body. Because of its ability to slip through block walls, Haunter is said to be from another dimension. They seem to eat by first paralyzing their opponent with Lick, then draining a bit of their life force, although it's possible they may eat other food as well, such things like Pokéblocks. It is also said to eat people's dreams by going inside them and scaring them to death. It is the lightest of all discovered pokemon.

Morty's Haunter was the second Pokemon sent out to battle against Ash.

Ash did not "catch" Haunter.
Haunter followed Ash to Saffron City.
Haunter stayed with Sabrina in Saffron City.

Haunter is apart of the Captain's Pokémon team who won the Orange Island League over 300 years ago.
The captain met his demise when a bad storm sank his pirate near Moral Island and the wreckage remained undisturbed until a couple of deep sea divers located the ship and scavenged the Orange Island League trophy which was placed in a museum.
Gastly and Haunter awakened from their Pokéballs and rose the ship above the water to search for the Orange Island trophy and replace it back inside the ship as a tribute to their late master.