From Bulbapedia, the community-driven Pokémon encyclopedia.

A Channeler (Japanese: きとうし Shaman) is a type of Pokémon Trainer that first debuted in the Generation I games. They are generally depicted as old Shinto priestesses. They scared out people from Pokémon Tower and were possibly unwittingly helping Team Rocket making their base in the tower. Their strange behavior when challenging the player and reactions after being defeated seem to imply they were possessed by the ghosts they used while battling. In fact, some Channelers apologize, saying "I was possessed!" when talked to after battle.

They specialize in Gastly and occasionally Haunter, them being two of the only three Ghost-type Pokémon available in Kanto.

In Pokémon Stadium, they're known as Mediums. In Generation II there is a Trainer Class with similar appearance named Medium. However, it's a different class.

Appearance

Channeler Sprites

Image from Pokémon Red and Blue

Image from Pokémon FireRed and LeafGreen

Generation I-introduced trainer classes
Beauty | Biker | Bird Keeper | Black Belt | Boss | Bug Catcher | Burglar | Champion | Channeler | Cooltrainer Cue Ball | Elite Four | Engineer | Fisherman | Gambler | Gentleman | Hiker | Jr. Trainer♂ | Jr. Trainer♀ | Juggler Lass | Leader | PokéManiac | Trainer | Psychic | Rival | Rocker | Rocket | Sailor | Scientist Super Nerd | Swimmer | Tamer | Youngster
Beta classes
Chief | Pokémon Prof.