Advanced Generation series
Treecko was caught in the episode Tree's a Crowd after Ash helped it in trying to save its old tree home; this event was followed by a battle with Ash's Pikachu, and Ash caught him as a result of that battle. In the next episode, Treecko struggled to make friends with the other Pokémon until it rescued May's Torchic from a wild Seviper, though it was badly hurt in the process. Later in the episode Treecko went off to do some special training on its own; when it had perfected a new variation of Pound in which it spun in the air before hitting the target, it battled the Seviper again, this time while under the ownership of Jessie, and won.
In The Winner by a Nosepass, Ash choose Treecko to battle Roxanne's Geodude where it quickly lost. Brock pointed out that even though Grass-types are strong against Rock-types, Treecko did not have an advantage due to it not knowing any Grass-type moves. In Brave the Wave it was used against Brawly, and while it managed to defeat Machop, it was still beaten by Brawly's Hariyama, which meant Ash lost the match. In Turning Over a Nuzleaf, when it and the other Pokémon were separated from their Trainers, Treecko would frequently taunt Corphish, though they later made amends in order to beat Team Rocket. Treecko battled Hariyama again in Just One of the Geysers and won, earning Ash the Knuckle Badge.
Treecko battled a Shiftry in Jump for Joy! to rescue Nurse Joy. Ash revealed in What You Seed is What You Get that the Shiftry had inspired him to try to teach Treecko how to use Bullet Seed. This became a reality when they met Natasha and her Grovyle, who helped Treecko learn the move using watermelon seeds.
In Going, Going, Yawn, Treecko was used in Ash's battle against Flannery. It was chosen to battle Slugma but was recalled after it was burned. It was sent out again to battle Flannery's Torkoal but eventually lost. In Gulpin it Down it ended up temporarily becoming giant alongside a wild Gulpin that had been terrorizing a large city.
Treecko evolved into Grovyle in the episode Exploud and Clear! during a battle against Guy, whose Loudred had evolved into Exploud at the same time. Ash has used Grovyle in almost every Gym battle since its evolution, including his battle against Norman in Balance of Power, where it battled Norman's Slaking and activated its Ability Overgrow. In Sky High Gym Battle!, Grovyle battled Winona's Altaria and won, despite the type disadvantage, and was later used against Winona's Swellow, though he lost. In Vanity Affair it once again battles an Altaria, this time belonging to the Hoenn Elite Four's Drake—with its incredible jumping ability, Grovyle was able to attack Altaria while it was in the sky, but was unable to defeat it. It was sent out to fight Juan's Luvdisc in The Great Eight Fate!. It was confused by Sweet Kiss and thrashed about, causing itself serious damage and fainting in the next episode.
Grovyle was used many times in the Ever Grande Conference. Its first battle was in Shocks and Bonds where it teamed up with Glalie to battle Quilava and Charizard, and though it was able to defeat Quilava, it lost to Charizard, leaving Glalie to battle on its own. In A Judgment Brawl, it battled Katie's Walrein and won the match for Ash, and in Ash's next battle against Morrison in Choose It or Lose It!, it beats Morrison's seemingly unstoppable Steelix only to be beaten by Morrison's speedy Gligar. In Ash's battle against Tyson in At the End of the Fray, it battled Metagross, and even after the damage Swellow had caused, Grovyle was unable to beat Metagross.
Kanto Battle Frontier
Grovyle battled Greta and her Hariyama in Wheel of Frontier, but was defeated.
In Odd Pokémon Out!, it had a crush on a Meganium owned by a Nurse Joy, and evolved into a Sceptile to rescue it when it had fallen into Team Rocket's hands. However, the Meganium was attracted to a Tropius, leaving Sceptile heartbroken and unable to use any moves. It regained its ability to use attacks in Cutting The Ties That Bind! to save Ash from Team Rocket with the use of Leaf Blade. It then used Quick Attack to save him and Bullet Seed to soften the fall. It was used in Ash's battle against Spenser in the following episode, Ka Boom with A View!, where it won Ash the Spirit Symbol after learning a new move, Solar Beam.
In Pokémon Ranger - Deoxys' Crisis!, Sceptile helped battle a wild Deoxys. It later battled another legendary Pokémon in Battling the Enemy Within!, Brandon's Regirock, when Ash was possessed by the evil spirit of the King of Pokélantis.
In Once More With Reeling! it fought in the Terracotta Contest final against May's Blaziken. The battle resulted in a tie where both May and Ash won the Ribbon.
Ash left Sceptile at Professor Oak's lab before setting off to Sinnoh.
Diamond & Pearl series
In The Semi-Final Frontier!, Ash's Sceptile had its third battle against a legendary Pokémon when it was sent out against Tobias's Darkrai in the semi-finals of the Lily of the Valley Conference. It used its newly-learned attack, Leaf Storm, to attack Darkrai, but was hit by its Ice Beam first. Soon, Sceptile was hit by a Dark Void and was put to sleep, but was able to wake up almost instantly, shocking Tobias, and defeated Darkrai with a powerful Leaf Blade. Sceptile was the only Pokémon in the Sinnoh League (or the region for that matter) to defeat Darkrai. However, after all the damage it had taken it was unable to defeat Tobias's next Pokémon, Latios, whose Giga Impact had overpowered Leaf Blade, knocking Sceptile out.
Best Wishes series
Sceptile was reunited with Ash in The Dream Continues!, where it and the rest of Ash's Pokémon posed in a group photo with him.