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. 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 capture him. But once he was captured, 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, to which he chose 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.
In Battling For The Love of Bug-Types!, Sewaddle was boldly sent out to fight Burgh and his Dwebble, after Tepig had already been defeated. Like in Tepig's battle, Sewaddle's attacks were all countered by Dwebble's Protect and he got hit with a powerful X-Scissor attack. Faced with an approaching Rock Wrecker, Sewaddle used String Shot to dodge by pulling himself towards the ceiling along the thread. After using String Shot to redirect another Rock Wrecker from his path, Sewaddle met another X-Scissor attack with Tackle and knocked Dwebble out of its rock. While Dwebble wasn't able to defend itself, Sewaddle hit it with a powerful Razor Leaf, Tackle and a second Razor Leaf, defeating Dwebble. Sewaddle stayed in the battle to fight Burgh's Whirlipede next. He continued to make use of his dodging technique with String Shot and the wrapped Whirlipede up in the thread. However, Whirlipede effortlessly cut through the thread and hit Sewaddle with a strong Steamroller attack twice in a row, tearing through Razor Leaf in the process. Sewaddle next attempted Bug Bite but was knocked back by Iron Defense and hit by yet another Steamroller attack. Sewaddle was next hit by Whirlipede's SolarBeam, which seemed as if it would defeat him until Sewaddle evolved into Swadloon. Swadloon wrapped Whirlipede up with String Shot as it did before, however, despite the move appearing much stronger than before, Whirlipede was still able to cut through the thread and attack. Swadloon attempted his dodging technique once more but Whirlipede cut through the thread attached to the ceiling, letting Swadloon fall. Swadloon was still able to stop Whirlipede's attack with his newly learned Energy Ball and finish it of with Tackle. Swadloon's third opponent was Leavanny and he was wrapped up in a very think String Shot. Ash cleverly told Swadloon to hit himself with Razor Leaf, damaging himself but freeing him from the thread. However Swadloon was quickly defeated by powerful Leaf Storm and Hyper Beam attacks.
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 wildDurant 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.
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 to battle. He was stubborn, and wouldn't admit to 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 when she suggested Ash get rid of him.
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.