Casey first appeared with her Chikorita in The Double Trouble Header. It had just helped her to catch a Rattata when she met Ash and his friends. She challenged Ash to a Pokémon battle. Ash defeated all three of her Pokémon including Chikorita with his Charizard. Later, Casey used Chikorita again to battle Ash's Pikachu and, thanks to the type advantage, Chikorita was able to resist Electric-type attacks. The battle ended without a winner being declared, however, because Team Rocket attacked both Trainers during the match and captured all of the Pokémon. In the end, Chikorita joined Pidgey, Rattata, and Ash's Pokémon to send Team Rocket blasting off.
Chikorita reappeared in a flashback in The Chikorita Rescue.
In The Bug Stops Here, Chikorita was used during the Bug-Catching Contest, despite having a disadvantage in the competition. Throughout the episode, Casey pushed Chikorita too hard and failed to notice when it became too tired to continue battling, forcing Ash, Misty, and Brock to call her out for this. When they were attacked by Team Rocket, Casey used her Chikorita again, and during the battle, it evolved into Bayleef and sent Team Rocket blasting off. Despite everything, Casey and Bayleef were unable to win the competition.
When Ash and his friends met up with Casey again in Here's Lookin' at You, Elekid!, her Bayleef was now fully evolved into a Meganium. During the search for Ash's Larvitar, Meganium was one of the Pokémon affected by the power of James's newly acquired Weepinbell. Casey used Meganium to defeat Jessie's Arbok, but despite its efforts, it was no match for Jessie's Pokémon. Later, Casey used Meganium in a battle against a wild Elekid. Although Elekid was powerful, it was defeated by Meganium, allowing Casey to catch it.
In A Date with Delcatty, Meganium was beside Casey as she was trying to find out about a mysterious note that Misty received. Later, Meganium was used alongside Misty's Corsola in a Tag Battle against Butch's Mightyena and Cassidy's Sableye. Meganium and Corsola proved to be very powerful and easily defeated their opponents.
In Those Darn Electabuzz!, Meganium joined Casey's Beedrill and Elekid to stop a runaway Charizard from advancing. Despite their strength, the trio couldn't keep the Fire-type Pokémon from away. Meganium later appeared with the other Pokémon to cheer on and help train Corey Demario's own Pokémon.