Diamond & Pearl
In Gettin' Twiggy With It! Ash originally caught Torterra as a Turtwig. It had been living in the forest near Clara's home, and it broke up fights among wild Pokémon in the area. After Team Rocket stole Ash's Pikachu, their balloon crashed, and Turtwig defended Pikachu against them. When Ash and his friends appeared, Turtwig assumed they were trying to wrongfully steal Pikachu, too, so it attacked them and ran off with an exhausted Pikachu in tow. The next time they confronted Turtwig, the misunderstanding was explained, and Turtwig apologetically befriended Ash. At the end of the episode, Ash and his friends began to leave, and the old woman encouraged Turtwig to go with Ash, so it ran to challenge him to a battle. Ash accepted his challenge, and Turtwig and Pikachu battled. Ash then caught Turtwig.
As Turtwig, it was a brave yet kind Pokémon that has high confidence for its small size. It apparently had a strong jaw, as it was often seen latching itself onto people and Pokémon's heads, tails, hats, and appendages to attack or show affection. It has done this to Ash as well, the behavior somewhat resembling that of James's Victreebel, Carnivine and Cacnea. It has also been shown to be both incredibly fast and quite strong, such as the time when it used Bite on Roark's Rampardos's tail and threw it into a rock, after which Brock said, "Big things come in small packages!"
In Different Strokes for Different Blokes, Turtwig battled Paul's Chimchar and lost. Although it held its own in battle, it appeared to hesitate upon receiving Ash's command to dodge attacks.
Overall, it has taken part in five of Ash's Gym battles. As a Turtwig, it defeated Roark's Cranidos in their first battle, and in the rematch, defeated the newly evolved Rampardos, giving Ash his first Sinnoh Badge. It also battled against Gardenia's Turtwig during his second Gym battle. Although Gardenia's Pokémon had defeated Ash's during a battle in the Eterna Forest, he gained the upper hand and defeated the Grass-type. After learning Energy Ball from Kenny's Breloom, Turtwig didn't battle again until the Pastoria Gym battle in A Crasher Course in Power!, where it battled Crasher Wake's Quagsire, but it was called back before being defeated.
Like many of Ash's Pokémon, Turtwig's battle style was based on its speed, a factor that Ash focused on even more after seeing Gardenia's Turtwig in action. However, in Aiding the Enemy! Turtwig evolved into Grotle during a battle against Paul's Honchkrow. It quickly became apparent that, due to its new body being larger and heavier, Grotle's speed had diminished, and Grotle fumbled and fell down a lot in the battle. The newly-evolved Grotle had high expectations of itself, well exemplified in the same episode. During the night, it went out to try to improve its speed. However, it failed to regain its speed and stumbled over from time to time. This was initially upsetting but Paul's Torterra provided some unexpected help by showing Grotle that it could rely on defense instead.
In Jumping Rocket Ship!, it was shown that Grotle became fatigued from walking a lot and from its increased weight. It had to be carried by Brock's Happiny for a portion of the journey.
In A Breed Stampede!, Ash used his Grotle to help train Dawn's Piloswine, where it used the defense method shown by Paul's Torterra. By Leading A Stray!, the defense method has begun to pay off in battle as Grotle took Swampert's Focus Punch on the head and replied accordingly with Razor Leaf, knocking its opponent out.
In The Lonely Snover, in order to save a Snover, Grotle surpassed its own limits and learned Rock Climb through its willpower alone, which allowed it to regain some of its speed.
In Sliding Into Seventh!, it was used in the Gym battle against Candice, its first appearance in a Gym battle since evolving. Using the lessons taught to it by Torterra, it stood firm against Sneasel's Icy Wind and was able to defeat her using Rock Climb which allowed it to easily travel across the ice without slipping. Unfortunately, it was easily defeated by her Abomasnow with a shattering Wood Hammer later in the battle.
In Pedal to the Mettle!, Grotle was used in the match between Ash and Paul and faced Honchkrow again. Grotle attempted to utilize its strong defense against Honchkrow, however Honchkrow proved to be far too powerful and as a result Grotle was defeated.
In Challenging a Towering Figure!, it was used to battle Palmer's Rhyperior in the Twinleaf Festival. During the battle it accidentally swallowed its own Energy Ball, causing it to gain a huge amount of power. After this boost, it was strong enough so that Rhyperior couldn't stop Grotle continuously pushing it backwards without slamming its tail into the ground. Despite then putting up a close fight, it lost the match.
In Gone with the Windworks!, it battled Dawn's Cyndaquil where the newly hatched Pokémon proved to be a strong battler.
In Dressed for Jess Success!, Grotle trained with Dawn's Mamoswine for the Lilypad Contest. During the training, Grotle demonstrated swallowing its own Energy Ball to gain power. Mamoswine mirrored the motion by swallowing its Ice Shard, which in turn caused ice to form on its back as an appeal.
In The Fleeing Tower of Sunyshore!, Grotle was used in Ash's first Gym battle against Volkner's Luxray where, despite the type-advantage, it started to get overwhelmed before the battle was canceled, due to the Gym losing power. Later, Grotle was called on to battle Team Rocket inside the Sunyshore Tower, and quickly evolved into a Torterra. Along with its new and more powerful form, Torterra immediately learned Leaf Storm and sent Team Rocket blasting off.
Torterra also appeared in An Elite Coverup! in a battle against Bertha's Hippowdon. After putting up a good fight, it eventually lost the battle.
In The Eighth Wonder of the Sinnoh World!, Ash used Torterra during his rematch with Volkner, sending it against Volkner's Electivire. Despite a type-advantage, Torterra was defeated.
Turtwig made a cameo appearance in League Unleashed!. When Ash was remembering his Gym battles, it was shown battling against Roark's Rampardos.
Ash used it in his battle against Paul in A Real Rival Rouser!. Torterra was sent out against Paul's Drapion, after both Buizel and Staraptor fell in battle against the powerful Pokémon. It battled well and was able to do some damage to Drapion with its Leaf Storm, Rock Climb and Energy Ball attacks. However in the end, Drapion proved to be too much for Torterra, and it was defeated by Drapion's powerful Pin Missile attack.
Ash left Torterra at Professor Oak's Laboratory when he decided to go to Unova.
Best Wishes series
Torterra was reunited with Ash in The Dream Continues!, where it and the rest of Ash's Pokémon posed in a group photo with him.