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.
Chikorita developed what seemed to be a crush on Ash, constantly fighting for his attention and trying to get his approval. In Gym Battles, she lost to Falkner's Hoothoot and defeated Bugsy's Spinarak.
In Current Events, Chikorita evolved into Bayleef to protect Ash from Team Rocket. However, it took a while for the amorous Pokémon to realize that she had grown so much - she continued to act as though she was still small enough to leap into Ash's arms for quite some time.
In Ash's Gym Battle against Chuck Bayleef defeated his Poliwrath and Machoke. At the Silver Conference she lost to Gary's Blastoise despite a type advantage, but made up for it the next day by defeating Harrison's Houndoom, against whom she had a type disadvantage. Ash left Bayleef with Professor Oak when he left for Hoenn.
Bayleef's personality is stubborn, sometimes violent, and passionate. She easily gets jealous of the attention Ash's other Pokémon receive - especially Pikachu. Some of her more extreme personality traits seemed to have mellowed with evolution, however.