Agatha
Agatha shows up in the anime as a temporary Gym Leader running the Viridian Gym in "The Scheme Team." She's best known for being a member of the Elite Four, though she doesn't advertise it at first.
Ash encounters her while passing through Viridian City on his way back to Pallet Town. After running into Scott, Ash gets competitive and decides to challenge Agatha to a gym battle to prove how strong he's gotten—he was riding high after making the top eight at the Ever Grande Conference. The battle doesn't go his way though; Agatha's Gengar takes him down.
The loss stings, but both Agatha and Scott are actually impressed by how well Ash fought. That's when Agatha drops the reveal: she's an Elite Four member. She and Scott both tell Ash he should head to the Battle Frontier and test himself against the Frontier Brains if he wants a real challenge.
Content compiled by AnimeList.moe from publicly available sources.
