As a Cyndaquil, in its debut
Quilava first appeared as a Cyndaquil in Good 'Quil Hunting. On the way to Azalea Town, Ash heard from a Trainer named Koji that the forest he and his friends were traveling through was populated with Cyndaquil. Ash decided to search for one, but he was separated from his friends and chased into a cave by Team Rocket. While running through the cave Ash found Cyndaquil. The two of them were attacked by Team Rocket's mecha, but Cyndaquil unleashed a powerful Flamethrower to defeat it. The mecha began to explode, so Ash threw a Poké Ball to catch Cyndaquil in an effort to protect it from the blast. Cyndaquil was successfully caught, even though Ash hadn't really intended to do so. Koji then reappeared and challenged Ash to a battle for ownership of Cyndaquil. Even though Misty and Brock told Ash that Cyndaquil was rightfully his and he didn't have to battle for it, Ash accepted the challenge. 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 the Pokémon used a Flamethrower at him.
Cyndaquil coming down for an aerial attack on Scyther
In Gettin' The Bugs Out!, Ash used Cyndaquil as his first Pokémon in his Azalea Gym Battle, going up against Bugsy's Spinarak. Although Misty thought Cyndaquil was a good choice, it was tired and its flame wasn't ignited. After using its agility to dodge Spinarak's String Shot, Bugsy had Spinarak used String Shot on the whole field, getting Cyndaquil's feet stuck. Ash tried to get Cyndaquil to use Flamethrower, but it didn't work, with Ash noticing that it must take time for Cyndaquil to warm up. Spinarak then hit Cyndaquil with Poison Sting, just as it was about to hit Cyndaquil with another one, Ash recalled Cyndaquil back and chose Chikorita. Later it went up against Bugsy's last and most powerful Pokémon, his Scyther. Cyndaquil was still unable to light its flame, and Scyther used Fury Cutter, which was easily dodged by its agility. Scyther then used Double Team, and Cyndaquil finally became able to use its flames and launched Flamethrower, destroying all the clones. Despite this the real Scyther used Swords Dance like a fan to block Flamethrower. It then hit Cyndaquil with Slash. After realizing normal attacks wouldn't work, Ash came up with an idea. Scyther prepared to use Fury Cutter but Cyndaquil used Flamethrower. Bugsy called it a bad idea and used Swords Dance, but Ash then had Cyndaquil jump instead and use Flamethrower directly above Scyther as the Swords Dance fan wouldn't work. It then finished Scyther off with Tackle, winning Ash the battle and earning him the Bug Badge.
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.
As a Cyndaquil with its flames ignited
In A Goldenrod Opportunity, Cyndaquil easily took down Whitney's first two Pokémon. However, Whitney's Miltank proved far too powerful and knocked out Cyndaquil with a trio of Rollouts. In the rematch, it lost to Miltank again, but managed to tire it out so that Pikachu could finish the battle. Against Morty in From Ghost to Ghost, it defeated Morty's Gastly with Tackle, since Gastly was still vulnerable to physical attacks from Noctowl's Foresight. Later, Cyndaquil was stuck battling against Haunter after it used Mean Look. It managed to strike with Swift and counter Hypnosis with a Smokescreen, but Haunter plucked Cyndaquil from the safety of the smoke and used Lick. It then tossed it to the ground, eliminating it from the battle.
It took part in Ash's Olivine Gym Battle in Nerves of Steelix! when Cyndaquil took down Jasmine's powerful Steelix. Steelix used Sandstorm to block Flamethrower, and a series of Digs and Iron Tails to attack Cyndaquil. However, Cyndaquil managed to hide in the holes dug by Steelix and took it down with an inferno fed by a sustained Flamethrower, winning Ash the battle and earning him the Mineral Badge.
Cyndaquil was used in Ash's gym battle against Pryce in Nice Pryce, Baby! where it battled and defeated Dewgong but was easily beaten by Piloswine's Blizzard, whom it had rescued from its icy tomb in the previous episode.
Ash used Cyndaquil in a battle against Satchel in You're a Star, Larvitar! where it battled his Magby but was defeated with Dynamic Punch. This loss along with Satchel's comments made Ash consider evolving Cyndaquil but this didn't happen.
Cyndaquil helped Ash get through the screening processes of the Silver Conference in A Claim to Flame!, and was entered in battle against Jackson at Misty's recommendation, in order to counter his powerful Meganium. However it didn't get to fight Meganium, losing to Jackson's Shiny Magneton's Thunder Wave and Tri Attack combination.
Ash left Cyndaquil along with the rest of his Johto team at Professor Oak's Laboratory when he wanted to start his journey fresh in the Hoenn region.
During Showdown at the Oak Corral, Cyndaquil was seen relaxing on the ground near Professor Oak's laboratory building. Cassidy stepped on Cyndaquil while she was approaching the building's window and Butch sprayed it with a fire extinguisher. Cyndaquil was not badly harmed and later helped defeat the two Team Rocket members using a Flamethrower attack commanded by Professor Oak.
It reappeared in The Right Place and the Right Mime, where it greeted Ash's Torkoal.
Quilava without its flames lit
Ash called Cyndaquil back into his party for the Lily of the Valley Conference in An Old Family Blend!. It was separated from Ash along with Snorlax, Bayleef, Totodile, and Corphish when Team Rocket attempted to steal them. While trying to find and reunite with Ash, Cyndaquil was attacked by one of Team Rocket's mechas. In the battle against Team Rocket Cyndaquil demonstrated that it had learned the move Flame Wheel during its time at the lab. Despite its new attack, Cyndaquil was having trouble defeating the robot. Cyndaquil then evolved into Quilava and used its new Eruption attack to destroy their mecha.
In the following episode, Quilava was the second Pokémon Ash used to battle Nando in the first round of the Lily of the Valley Conference after Staraptor. Quilava went up against Nando's powerful Armaldo, and during the battle it was revealed that it had learned Aerial Ace. When Quilava's Aerial Ace and Armaldo's X-Scissor hit each other, Armaldo went down first. Quilava smiled to itself but then fainted, thus both Pokémon were knocked out.
Quilava was reunited with Ash in The Dream Continues!, where it and the rest of Ash's Pokémon posed for a group photo with their Trainer.