Quilava first appeared as a Cyndaquil in Good 'Quil Hunting. On the way to Azalea Town, Ash hears from a Trainer named Koji that the forest, he and his friends were traveling through, was reputed to be populated with Cyndaquil and decides to look for one. Ash was separated from his friends and chased by Team Rocket into a cave where he found Cyndaquil. It was attacked by a giant Team Rocket's mecha, but Cyndaquil unleashed a powerful Flamethrower to defeat them. Ash threw a Poké Ball at Cyndaquil, hoping to capture it in order to protect it from the explosion, which he succeeded in doing. Koji then challenged Ash to a battle for Cyndaquil, but Brock told Ash that Cyndaquil was his by right and he didn't have to battle for it. Ash accepted the challenge nonetheless, and Cyndaquil, despite having a disadvantage, was able to use its speed and Tackle attack to defeat Koji's Sandslash. Koji then decided to try to steal Cyndaquil anyway in a net, only to get badly burned when it used a Flamethrower at him, causing him to run away.
Cyndaquil's Gym battle debut came quite early, used only three episodes later in Ash's battle withBugsy. Ash was confident that Cyndaquil could provide the firepower needed to take down the Bug-type gym, but when Cyndaquil's fire didn't come out when called upon, Bugsy's Spinarak landed several Poison Stings and then tied Cyndaquil up with a String Shot. Ash recalled Cyndaquil, and only sending it out as his last resort against Scyther, and was once again put at a disadvantage against Scyther's Slash and its use of Swords Dance to deflect Flamethrower. However, it won the battle by leaping over Scyther's head, hitting it from above, where Scyther was unable to dodge.
Cyndaquil with its flames ignited
Cyndaquil's sluggish battle skills were overcome in Hot Matches!, where it was outclassed by Miki's Skarmory. Ash trained with Cyndaquil, raising its attack and speed, and it also learned how to ignite the flames on its back at will whilst fending off wild Beedrill. In a rematch, Cyndaquil took down Skarmory with Flamethrower.
In a Christmas special, it was seen helping Delibird find his presents and helped to build the Snorlax Snowman. During the special episodes, Cyndaquil was seen laying on the floor. It was trod on by Cassidy and was squirted with foam by Butch.
Personality-wise, Cyndaquil was the "baby" of Ash's group. It started off very weak and lazy, and was easily tired out. In battles, it usually had to wait for the flame on its back to ignite before using any Template:Type2 attacks. After training with Ash in Hot Matches!, Cyndaquil was able to ignite at will. Its evolution into Quilava marked a significant spike in its maturity and skill level.
Quilava, as a Cyndaquil, took the longest to evolve out of any Pokémon in the anime, having been in its first stage for 507 episodes, longer than a decade. However, it took less than a decade for the dub.