There is one Gym Leader from the Kanto region that has yet to appear in the Pokémon anime, even though she debuted in the games over twenty years ago. Ash Ketchum battled all of the Kanto Gym Leaders during his first journey, with the exception of Giovanni, who allowed Team Rocket to stand in for him.

The Gym Leaders in the Pokémon anime are the same ones from Pokémon Red and Blue, except for Team Rocket. The Pokémon anime expanded some of them a great deal, with Brock and Misty being Ash's first two companions on the road. In the games, only Giovanni had a role in the story outside of being a Gym Leader. This gradually changed over time, with future Pokémon games giving the Gym Leaders more of a role in the story. This culminated in all of the Galar Gym Leaders showing up for a final tournament in Pokémon Sword and Shield. 

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In Pokémon Gold and Silver, the player is given the chance to return to the Kanto region and perform a second Gym challenge. Six of the original Gym Leaders return, all with aged-up designs and a stronger group of Pokémon. There are new faces among the Kanto Gym Leader line-up, as Blue (the rival from Pokémon Red and Blue) has taken over the Viridian City Gym. The Fuschia City Gym also has a new Gym Leader, as Koga (the former Gym Leader) joined the Elite Four. Koga was replaced by his daughter Janine, who also specialized in Poison-type Pokémon.

Once Ash returned from the Orange Islands in the Pokémon anime, he started his second journey in the Johto region. Ash has periodically returned to the Kanto region after the end of each of his adventures, but he has yet to return to Fuschia City. As such, Janine has never appeared in the Pokémon anime. It's not even clear if Koga ever joined the Elite Four, as time in the Pokémon anime is static, with Ash, Brock, and Misty being the same age they were at the start of the series. Janine might not have ever become a Gym Leader.

There's a chance that Janine's day may come. The Pokémon Journeys season involves Ash traveling to different regions, as he is taking part in the World Coronation Series. He has already returned to at least one Gym in the Kanto region, as he battled Lt. Surge's replacement in the Vermilion City Gym. It's possible that Ash might need to travel to Fuschia City to battle his next opponent, who turns out to be Janine, finally giving them the chance to have a Pokémon battle.

Next: Pokémon: Is Lt. Surge Still An American