Leavanny first appeared as a Sewaddle while Ash and his friends were in Pinwheel Forest. He randomly attacked Ash's Pikachu, and Ash tried to catch him, but was not successful, since Sewaddle ran away. He later reappeared and knocked Ash to the ground when the trio met Burgh, the Gym Leader of the Castelia Gym, who revealed that he was accompanying Sewaddle through the forest, even though he was not his Pokémon. When Ash and his friends tried to say hello to Sewaddle by using the two sensory knobs on the top of his head, he Tackled Ash, while he kindly said hello to Iris, Pikachu, Axew and Burgh. Ash, Iris and Burgh then followed Sewaddle throughout the forest, watching him eating, as well as watching his sleeping habits.
When Sewaddle started eating some fruit, two Woobat started to attack him. Ash protected Sewaddle from an Air Slash attack, and Pikachu and Sewaddle then scared off the Woobat. Sewaddle then offered Ash some fruit, showing that he was slowly beginning to like him. Later that night, Sewaddle even went into Ash's sleeping bag with him. The next day, a Patrat kidnapped Sewaddle in order to heal its sick friend, as a leaf from a Sewaddle has healing properties. However, Sewaddle got away, just to get stuck on a Deerling's neck, and to get thrown over a cliff into a river, with Ash diving in to save him. Burgh then used his Leavanny to save both Sewaddle and Ash from going over a waterfall. Sewaddle was very thankful, but his "clothes" got messed up during the fall. However, Burgh's Leavanny made him new clothes. Ash asked Sewaddle if he wanted to join him, and Sewaddle happily allowed Ash to catch him. But once he was caught, his Poké Ball became sealed, as Ash had seven Pokémon on him at the time. This meant Ash had to send over one of his Pokémon to Professor Juniper in order to retain Sewaddle in his party, which was later revealed to be his Pidove.
In A Connoisseur's Revenge!, Ash wanted to have Sewaddle examined by a Pokémon Connoisseur so he would know how compatible he was with him. He showed him to a C-class Connaisseuse by the name of Burgundy. She confirmed that his Ability is Swarm, which she thought was a bad thing, as she thought Ash would be better off with a Sewaddle with Chlorophyll, not doing anything to support her claim. When she said he should replace Sewaddle, he fired a String Shot at her in anger.
Ash used his Sewaddle as his second Pokémon in his Castelia Gym battle in Battling For The Love of Bug-Types!. Sewaddle went up against Burgh's Dwebble, but Ash did not mind because he wanted to use a Bug-type despite the type disadvantage since Sewaddle was part-Grass-type. Sewaddle used Razor Leaf, which was blocked by Dwebble's Protect. Dwebble then used a fast X-Scissor, but Sewaddle dodged it by aiming String Shot onto the roof and lifting himself up to dodge. He then used Bug Bite, which was countered by Protect. Dwebble then went for a close-range Rock Wrecker, like it used to knock out Ash's Tepig, but Sewaddle dodged it by aiming a String Shot at the wall. He followed up with Razor Leaf, which was countered by Protect again. Dwebble used Rock Wrecker, but Sewaddle used String Shot to grab and throw the Rock Wrecker. This prompted Dwebble to use another X-Scissor, which was countered by Sewaddle. This separated Dwebble from its shell, allowing Sewaddle to hit it with Razor Leaf. When it tried to return to its shell, he hit it with his Tackle and then defeated the Rock Inn Pokémon with his Razor Leaf.
Burgh then sent out his Whirlipede, and Ash decided to stick with Sewaddle despite Whirlipede being a Bug/Poison-type. Whirlipede went for Poison Sting, which was dodged when Sewaddle used String Shot on the roof. He then used String Shot on Whirlipede, but these threads were easily cut through by Whirlipede's spinning. Whirlipede then used Steamroller, hitting Sewaddle. It used it again after Sweddle unsuccessfully attempted to counter with Razor Leaf. Sewaddle attempted to use Bug Bite, but this was blocked by Iron Defense, hurting Sewaddle. After being hurt by Steamroller again, Whirlipede used Solar Beam, which hit Sewaddle, but he endured it and immediately evolved into a Swadloon. Swadloon used String Shot on Whirlipede, but despite being more powerful, the threads were easily cut again by Whirlipede's spinning. It then used Steamroller; Swadloon attempted to dodge by using String Shot on the roof, but Whirlipede cut those threads and continued on with Steamroller. Swadloon then used his new move, Energy Ball, to stop it and deal damage. He then defeated the Curlipede Pokémon with Tackle.
Burgh then sent out his last Pokémon, Leavanny, the evolved form of Swadloon. He started off with Energy Ball, but this was countered by Leavanny's Protect. Leavanny then used String Shot on the Leaf-Wrapped Pokémon, trapping him. However, Ash got Swadloon to cut the threads with his own Razor Leaf, freeing him but dealing damage. Unfortunately, this was in vain as the Nurturing Pokémon used Leaf Storm and finished off Swadloon with Hyper Beam.
In Battling the Leaf Thieves!, Swadloon battled against Iris's Emolga in a training battle. After being hit hard by a Discharge attack that Emolga sent down the thread of Swadloon's String Shot, he perked up and evolved into Leavanny. His innate caring nature came to force right away and Leavanny made leaf hoods for the young Axew and Scraggy, who were very pleased with them. Leavanny then noticed three wild Sewaddle who all had withered hoods. Not giving it a second thought, Leavanny made new hoods for the three Sewing Pokémon. After making more hoods for Pikachu, Oshawott, Tepig, Snivy and Emolga, the gang sat down for a nap. While sleeping a group of wild Durant came across the gang and took the leaf hoods for food as well as Axew, who was still wearing his outfit, because their poor eyesight led them to believe he was a pile of leaves. When the gang awoke, Leavanny helped search for the missing Pokémon. After Emolga led the gang to the entrance of the Durant's maze, Leavanny cleverly lined the cave with thread behind the group to mark the way out of the maze. Once they found the Durant's nest, Leavanny created leafy cocoons for Ash and Scraggy so they could retrieve the still sleeping Axew. Leavanny joined the others in fighting the Durant after Ash revealed himself, however the team's moves were all blocked by Protect. After Pikachu, Scraggy and Emolga managed to defeat the swarm, Leavanny led the gang back through the maze and to safety.
In Rocking the Virbank Gym! Part 1, Ash chose Leavanny as his third Pokémon in his Gym battle with Roxie, whose Koffing had already defeated Ash's Boldore and Unfezant. Though Ash knew Grass-types were at a disadvantage against Poison-types, he was not discouraged. Leavanny first attacked Koffing with Energy Ball, which scored a direct hit, both damaging Koffing and lowering its Special Defense. Koffing tried to counter with Gyro Ball, but it was quickly stopped by Leavanny's String Shot. Ash had Leavanny spin Koffing around with the thread, preventing it from attacking him, until he finally fired another Energy Ball, which was enough to finally defeat the Poison Gas Pokémon. Roxie next sent out her Scolipede. Leavanny's Razor Leaf didn't seem to do a thing against the giant Bug/Poison-type, and Roxie had Scolipede use Toxic on Leavanny, badly poisoning him. In the next episode, Leavanny fired an Energy Ball attack, but Scolipede dodged it easily. It then attacked Leavanny with Rock Tomb and Sludge Bomb, which, added with the damage caused by the worsening poison, was enough to defeat the Nurturing Pokémon.
In Jostling for the Junior Cup!, Leavanny was used in the first round of the Pokémon World Tournament Junior Cup against Marris's Karrablast. He was able to knock Karrablast out with a powerful Energy Ball attack and win the battle. Thus, Ash advanced to the second round.
In Strong Strategy Steals the Show!, Ash used Leavanny as his third Pokémon in his battle against Stephan during the Vertress Conference, sending him out against Stephan's Sawk. Although Leavanny was able to avoid Sawk's initial attempts of Close Combat and Karate Chop, as well as hit him with his new X-Scissor attack and capturing Sawk with String Shot but failing due to Sawk being powered up by Bulk Up, Sawk soon gained the upper hand by using Low Sweep, lowering Leavanny's speed and preventing him from dodging a barrage of Close Combat's which caused serious damage on Leavanny. This, however, caused Leavanny's Swarm Ability to activate, giving him a big power boost on his Bug-type moves. Using this new power, he launched a String Shot on Sawk in which Sawk was unable to break, even with Bulk Up, he then used a lot of powered up X-Scissor attacks on Sawk, but Stephan used this to his advantage by having Sawk use it to break the threads, then, after a hard-fought battle, Leavanny was finally brought down by a Bulk Up-powered Karate Chop from Sawk.
In Farewell, Unova! Setting Sail for New Adventures!, Professor Juniper sent Leavanny to Professor Oak's Laboratory before Ash left for Kanto, touring through the Decolore Islands. Ash called upon Leavanny in Butterfree and Me!, so he could help Ash evolve a wild Caterpie into a Butterfree. Leavanny was also shown in one scene making clothing for Caterpie. Leavanny later reunited with Ash and the entire Unova team at Oak's lab in The Dream Continues!. Leavanny joined in with them to launch a group attack on Team Rocket, using Energy Ball to send them blasting off for the final time in the Best Wishes series. Leavanny later posed in a group photo with the rest of Ash's Pokémon. Then, he was left at the lab when Ash set off for the Kalos region.
As a Sewaddle, Leavanny was childish, often doing things without warning when Ash first met him, such as attacking Pikachu for no reason other than wanting to battle. He was stubborn, and wouldn't admit his feelings for Ash right away, but after he had been saved a few times, he became more comfortable with Ash, and snuggled up to him. Since then, he had been very caring towards his Trainer, even to go so far as to wrap Burgundy's head in a String Shot web merely one episode later when she suggested Ash get rid of him.
Leavanny, as both a Sewaddle and Swadloon, was also competitive since his debut, and has a strong passion for battling. He always tried his hardest, allowing him to defeat two Pokémon he was at a strong disadvantage against (Burgh's Dwebble and Whirlipede). This also allowed him to evolve into Swadloon. Since he has evolved, Swadloon is a bit more mature, although he still got a bit childish when he believed Oshawott, Tepig and Scraggy took his food in Emolga and the New Volt Switch!. However, he still has a passion for battling. Swadloon also has some sewing skills, as seen in Movie Time! Zorua in "The Legend of the Pokémon Knight"!, where he helped make costumes for Luke's movie with his Leavanny.
After evolving into Leavanny, he adopted a more caring nature and acts like a parent to the other Pokémon. He also became very independent, prancing off on his own to find leaves to use for clothes, taking a lot of pride in his work. He is also able to make good use of his newly acquired skills, cleverly lining the Durant's maze with thread to mark the way out without being commanded to by his Trainer. As shown in Rocking the Virbank Gym! (Part 1) and Strong Strategy Steals the Show!, he has also been able to use these new skills in battle to powerful effect.