Gym Leader

A Gym Leader (ジムリーダー) is the highest-ranking member and owner of an official Pokémon Gym. Gym leaders typically only specialize in one specific type of Pokémon (the only known exception being Blue) and uses them to test the skills of trainers that challenge them. Every Gym Leader holds a unique badge for the trainers that prove their skills to them or defeat them in battle. When a trainer has a total of eight badges, they can compete in their regional League or challenge their region's Elite Four. There are a total of eight Gym Leaders in each region, usually all eight gym leaders in a region will specialize in a different type of Pokémon, the only type not appearing so far (Gen 1-6) being, although certain types are used for multiple gyms but in different regions, the most recurring type for the first gym is due to both  and  being super effective against it.

As shown several times, a Gym Leader is usually well involved with their community. When not at the Gym, a Gym Leader is usually involved in some type of work or service that the surrounding community is known for (this is more prevalent in the anime and the manga than the videogames). When at the Gym, a leader will maintain the Gym itself, the Pokémon, the other members, and will battle any challenger.

Kanto
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Hoenn
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Unova
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Alola
Alola has no Gym Leaders. Instead, the player character can battle the four Island Kahuna in a Grand Trial, after passing the Trial.

Pewter City
While Brock begins as the Gym Leader, his father: Flint, is the former Gym Leader, his mother: Lola took over the gym and made it a -type Gym, and his younger brother Forrest is the current Gym Leader.

Route to Vermilion City
While Ash travels and meets the Gym Leaders from the games as normal, he also meets a trainer named A.J., who runs his own unofficial Gym which is on the route to Vermilion City, A.J.'s gym does not have a badge or a type.

A.J. also makes a brief appearance in the games as a Youngster on Route 9.

Cerulean City
While Misty is the current Gym Leader, her three sisters: Daisy, Violet, and Lily act as Gym Leaders until Misty takes the post.

Viridian City
While Giovanni starts as the Gym Leader like in the games, he has to go on a business journey - so he leaves Jessie, James, and Meowth as the Gym Leaders. Jessie is the one who actually does the battling.

Since Giovanni does not return from the business trip as he is busy running Team Rocket, the Elite Four member Agatha is put in charge of the Gym.

Orange Islands
The Gym Leaders from the Orange Islands have only appeared in the Pokémon Anime series and not in game series. In addition to this, these Gym Leaders work slightly differently than others, usually having trainers that face them take part in a number of challenges rather than Pokémon Battles. Also, the majority of these Gym Leaders do not specialize in a specific Pokémon type.

Trivia

 * The only type of Gym Leader that is not present is Dark-type. However, there are Elite Four with type Pokémon.
 * In Pokémon Black and White 2 World Tournament, all of the Starter type Gym Leaders have a fully evolved starter type.
 * The only regions with Gym Leaders for all three of the starter types are Kanto and Unova.
 * However, in the Generation V games, the player character is only able to have a gym battle with the Gym Leader for one of those types (starter's weakness in Black and White; in Black 2 and White 2).
 * Johto is the only region with no Gym Leaders that specialize in any of the starter types - Fire, Water or Grass.
 * Due to the Hoenn and Unova regions having two -type gym leaders each (Wallace and Juan; Cress and Marlon), there are a total of six Gym Leaders of this type, more than any other type.
 * However, the type with Gym Leaders in the most regions is, with a Gym Leader of that type in every region except for Johto.