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.
In The Grass-Type Is Always Greener!, Turtwig was first sent out to rescue Cheryl, from vines with its Razor Leaf. It stayed out of its Pokéball and was later found by Gardenia who found it cute. It later battled as Ash's first Pokémon in his and Gardenia's unofficial two vs two battle. She chose her Cherubi. Turtwig used Tackle hitting Cherubi, it then used multiple Razor Leaf's hitting Cherubi the first time but Cherubi dodged it and Gardenia didn't give any commands. Impressed with Turtwig, she recalled her Cherubi, which Brock speculating Gardenia using Cherubi to check out Ash's Turtwig. Gardenia then used her Turtwig, Ash's Turtwig used a fast Tackle but Gardenia's was faster and it easily dodged. Ash's Turtwig used Tackle again but was dodged again crashing it into a tree. Gardenia's Turtwig used Leech Seed draining Ash's Turtwig. Whilst Leech Seed drained its health. Gardenia's Turtwig used an extremely fast Tackle, knocking Turtwig out.
In The Grass Menagerie!, Turtwig was Ash's first Pokémon in his Eterna Gym Battle against Gardenia, it went up against her Cherubi again. Turtwig started off with Tackle, which was dodged, its Razor Leaf was dodged as well, showing Cherubi's speed, this was because of Cherubi's ability Chlorophyll, this made Cherubi faster as it was very sunny. Cherubi then used Solar Beam, but Turtwig was able to dodge. Ash thinking that Solar Beam couldn't be activated straight away and had Turtwig use Razor Leaf, but Cherubi countered with an instant Solar Beam, which was instant due to the harsh sunlight, destroying Razor Leaf and hitting Turtwig, Cherubi then used Magical Leaf and Ash recalled Turtwig before the attack could hit. Later it was called out to face Gardenia's Turtwig again, Ash's Turtwig used Tackle which Gardenia's Turtwig dodged and used Leaf Storm, however Ash's Turtwig was able to dodge by jumping on and then off a tree. Ash's Turtwig used Bite but Gardenia's Turtwig used Leech Seed, stopping Bite and draining Ash's Turtwig of energy. Turtwig was able to withstand it and used Synthesis, however Gardenia had her Turtwig use Leaf Storm, which Turtwig was able to dodge by jumping, allowing the Synthesis to finish. Ash's Turtwig was then able to hit Gardenia's Turtwig with Razor Leaf, both Turtwig then used Tackle at each other, but only Ash's Turtwig was able to remain standing, winning it the rematch of Turtwigs. Ash kept Turtwig in but the Leech Seed was still in effect. It then went up against Gardenia's final Pokémon, Roserade. Turtwig used Tackle but the Bouquet Pokémon tripped up Turtwig with Grass Knot, Roserade then used Weather Ball, which was a Fire-type attack due to the harsh sunlight, defeating Turtwig.
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 Ash's first Pokémon in his Gym battle against Candice, going up against her Sneasel, using her skating skills, she dodged its Energy Ball and then used Icy Wind, however Grotle used the training it got from Paul's Torterra to endure these attacks despite being super effective, and then used a close range Razor Leaf against her Fury Swipes. It then used Rock Climb against Sneasel's Icy Wind knocking her out. Grotle was then recalled. It later went up against her Abomasnow, it used Rock Climb but this was countered by Wood Hammer knocking Grotle out.
In Pedal to the Mettle!, Grotle was used in the match between Ash and Paul. It initially faced his Electabuzz, but had an Energy Ball blocked by Light Screen before Paul switched it for Honchkrow, pitting the two against each other once more. Ash decided to keep Grotle in, however Honchkrow proved to be far too powerful with its type-advantage on top of Electabuzz's Light Screen still being active. A badly timed Synthesis then gave Honchkrow time to ready Sky Attack and as a result Grotle was defeated, becoming the first of Ash's Pokémon to fall in the battle.
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 as his first Pokémon during his rematch with Volkner, sending it against Volkner's Electivire. Torterra's Leaf Storm was blocked by Electivire's Fire Punch, this was blocked again in the same way but Torterra broke through Fire Punch by launching an successful Energy Ball, it then used Rock Climb which was countered by Electivire's super-effective Ice Punch which knocked Torterra out.
Torterra made a cameo appearance as a Turtwig in League Unleashed!. When Ash was remembering his Gym battles, it was shown battling against Roark's Rampardos.
Ash used it as his fifth Pokemon 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 immediately after the use of Synthesis.
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.