Bayleef was introduced in The Chikorita Rescue as a Chikorita. Chikorita was especially stubborn, going so far as to fight against Ash's Charizard, despite obvious overwhelming odds; she was injured in the process. Team Rocket tried to convince the stubborn Pokémon to join them, but Ash eventually won her confidence by saving her from Team Rocket.
Chikorita later was used in Once in a Blue Moon to try getting the GS Ball back. However, Chikorita only wanted to be affectionate, so when Quagsire got near Ash, Chikorita got angry and tried to hit it with her leaf but she accidentally hit Ash instead.
Chikorita made her Gym battle debut in Fighting Flyer with Fire, where she took on Falkner's Hoothoot. Ash showed that he believes in Chikorita by defending his choice to a disapproving Falkner, but Chikorita was outclassed by the Flying-type, who dodged her Vine Whip and Razor Leaf and then knocked her out with a Peck and a Tackle.
Chikorita's jealousy of Pikachu and resentment towards Ash arose in Chikorita's Big Upset. Ash tried to withdraw Chikorita for Pikachu in a battle with a Trainer's Raticate, but Chikorita would not allow this and the battle was suspended by the time Ash returned her. She later ran away during the night and stumbled into the territory of a group of urban Pokémon. Lashing out at the leader, she knocked it out and became the new leader. Team Rocket tried to take her and the other Pokémon with their robot made out of tires, but Ash risked himself to save her, and Chikorita returned the favor by catching him when he fell. The two then took down the robot and Chikorita forgave Ash and became friends with Pikachu.
In Gettin' The Bugs Out, Ash used Chikorita as his second Pokémon in his Azalea Gym Battle despite the type disadvantage, something which Bugsy mocked. She went up against Spinarak. After dodging some String Shots, she used Vine Whip, which Spinarak was able to dodge by aiming String Shot at a tree. Chikorita launched multiple Vine Whip attacks, all of which were dodged by Spinarak who used the trees to dodge. Her Razor Leaf and Spinarak's Poison Sting collided and destroyed each other. Chikorita then used Sweet Scent to distract Spinarak and then she defeated it with Tackle. She then went up against Metapod, Chikorita used Vine Whip but Metapod countered with Harden, and Vine Whip didn't faze Metapod. The same thing happened with her Razor Leaf. She used Razor Leaf again but Metapod jumped and used Tackle, knocking Chikorita out.
In The Totodile Duel, Chikorita defeated Misty's Staryu. She was almost defeated by the powerful Water Gun, but she dug in her heels and then managed to dodge a Tackle and hit Staryu from behind with Vine Whip. By this point, she had proven to increase her battling capability significantly, taking down Jessie's Arbok in a single Tackle in The Heartbreak of Brock.
She also appeared several times in Spell of the Unown: Entei, where she was used in the battle against Lisa. She put up a good fight against Girafarig, but was ultimately defeated by Psybeam.
With Ash, upon evolution into Bayleef
In Current Events, Chikorita evolved into Bayleef to protect Ash from Team Rocket. However, in the following episode Turning Over A New Bayleef the amorous Pokémon was shown not to have realized she had grown so much and continued to act as though she was still small enough to leap into Ash's arms. This problem was addressed when she ran away after Ash scolded her to "get away and stay away". Bayleef then came across Haruno and her Pokémon, and she and Ash made up in the process of taking down Team Rocket's giant Bayleef mecha. She learned Body Slam at the end of the episode.
In Ash's Gym battle against Chuck in Machoke, Machoke Man!, Bayleef single-handedly won Ash the Badge after Pikachu's defeat to Poliwrath. She bested Poliwrath in a display of strength, slamming Poliwrath to the ground with a Vine Whip and then using Body Slam to knock it out. She then fought Chuck's powerful Machoke, taking damage from Karate Chop and Submission, but showed good strategy by restricting Machoke's arms with her vines to block his Cross Chop, and then used her own Submission-induced momentum to in turn slam Machoke to the ground. She then used Body Slam and a point blank Razor Leaf to take out Machoke and win the battle.
She was briefly sent back to Professor Oak's lab in Dueling Heroes, in exchange for his Kingler, who Ash needed to compete in the Whirl Cup. She did not warm up to Oak well, and Oak called Ash to inform him that Bayleef was restless. Finished with Kingler, Ash returned Bayleef to his team in the next episode.
In Extreme Pokémon!, Bayleef participated in the Extreme Pokémon Race with Ash, racing against Gary Oak and his Arcanine. She showed an immense feat of strength by hurling Ash at Team Rocket's balloon so that Ash could pierce it with his skateboard and rescue Arcanine. She then arrived just in time to attack Jessie's Arbok and hold it off long enough for Gary to catch up, who took down Arbok with his Arcanine. Bayleef and Ash worked together to finish the race and they managed to best Gary just at the end, winning the race for Ash.
It was the only member of his then-current team to make up Ash's team of six against Gary Oak. In Can't Beat the Heat!, she was sent out to give Ash a type advantage over Gary's powerful Blastoise, managing to use Vine Whip to jump above Hydro Pump. However, her aerial Razor Leaf was deflected by a Rapid Spin, and she was knocked out when Blastoise bit her leaf and tossed her aside.
Ash's Bayleef leaping with Vine Whip
She redeemed this loss in Playing with Fire!, in a battle with Harrison's Houndoom, again showing innovativeness by clamping Houndoom's jaw shut with Vine Whip and then using Body Slam to take it down. She put up her best efforts against Blaziken, but was unable to overcome another type disadvantage and was defeated by a Fire Punch. She was left at Professor Oak's lab before Ash departed for Hoenn.
In special episodes, she was seen as a Chikorita and a Bayleef. As a Chikorita, she was seen helping Delibird to collect its presents and to make the Snorlax Snowman. As a Bayleef, she was seen helping Tracey to feed the Pokémon at the ranch.
Advanced Generation series
She was reunited with Ash in The Right Place and the Right Mime, where she rescued him from his stampeding Tauros, who had carried him off in their excitement.
Diamond & Pearl series
She appeared in An Old Family Blend! alongside Ash's other Pokémon. After briefly being stolen by Team Rocket, Bayleef escaped from her Ball and went looking for Ash. Eventually, Bayleef thundered out of the trees and tackled Barry. Then, she found Ash and tackled him, too. When he tried to return her to her Poké Ball, she took it in her mouth, wanting to spend more time with Ash. She later helped destroy the hands of one of Team Rocket's mechas, and afterwards, was sent back to Professor Oak's lab. Bayleef was not seen participating in the Lily of the Valley Conference.
Best Wishes series
Bayleef was reunited with Ash in The Dream Continues!, where she and the rest of Ash's Pokémon posed in a group photo with him in a post-credits scene. Though cut from the international episode, it was ultimately posted on the official Pokémon Facebook and Twitter accounts.