"Hello. Lovely weather isn't it? It's so pleasant. ...Oh dear... I must have dozed off. Welcome. My name is Erika. I am the Leader of Celadon Gym. I teach the art of flower arranging. My Pokémon are of the Grass-type. Oh, I'm sorry, I had no idea that you wished to challenge me. Very well, but I shall not lose."
Erika first appeared in the episode Pokémon Scent-sation, where she was first seen as a manager at a perfume shop. Due to an insult of the perfume from Ash, Erika banned him from the shop. When Ash and co. made their way to the local Gym, they found out that Erika was also the Gym Leader, where she banned him there too.
Following a plan made up by Team Rocket, Ash dressed up as a girl and re-introduced himself as "Ashley", so as to sneak inside the Gym. After his disguise was disintegrated by Pikachu, Erika later on decided to battle him. She had to under Pokémon League rules. Their match was cut short however, when Team Rocket decided to set fire to the Gym.
Everyone made it out of the Gym until Erika realized that her Gloom was still inside. Ash then ran into the gym and saved the Gloom. After the ordeal, Erika rewarded Ash with a Rainbow Badge for his actions.
In the manga
Red, Green and Blue Arc
Erika first told Red to catch an Eevee before he could challenge her at her gym.
After catching Eevee he challenged Erika and lost. Even though he did, Erika saw potential in Red and gave him the Rainbow Badge.
She fought against the mind controlled Pokémon that was aiming for all of the badges, and just like the other gym leaders, her badge was also taken.
Gold, Silver and Crystal Arc
She fought against Pryce during the competition that is a battle of two regions namely Kanto and Johto. Though she lost to Pryce.
In the game
Erika was in one of the first set of Gym Leaders in the original Pokémon games. She was again a Gym Leader in Pokémon Gold and Silver, and was updated in the Generation I remakes, Pokémon FireRed and LeafGreen. Erika's Gym is filled with grass, flowers and other plants, and additionally, all of the Gym members are female. The player would need to use Cut in order to reach Erika in the Gym's center.
Upon defeating her, Erika rewards the player with Rainbow Badge along with TM21 (Mega Drain), in the Generation I games. In the Generation II and III games she gives out TM19 (Giga Drain).
Gloom is Erika's main and favorite Pokémon. It was first seen absorbing Koffing's Smoke and knocking out Team Rocket with its stench. It was later seen in Ash's gym battle with Ash where it was the final Pokémon used in the battle. It quickly beat Charmander by releasing its strong smell. It was then caught in the fire in the gym caused by Team Rocket and Ash saved it which made Erika give him the Rainbow Badge out of gratitude.
Weepinbell was only seen shortly in battle. It went up against Ash's Charmander and attacked immediately with Razor Leaf but was beaten quickly when Charmander burnt all of the Razor Leaf and the Skull bashed Weepinbell out of the match.