As a Sewaddle and Swadloon, he was naughty and doesn't like to give up. However, after he evolves, he begins to like smaller Pokémon in the team and starts to take care of his friends.
Leavanny first appeared as a Sewaddle in Sewaddle and Burgh in Pinwheel Forest!, when Ash and his friends were travelling through Pinwheel Forest. Ash attempted to capture Sewaddle after it attacked Pikachu, but Sewaddle ran away before he could. Sewaddle knocked Ash to the ground when the gang met Burgh. It was then revealed that Burgh was accompanying Sewaddle through the forest.
Sewaddle initially disliked Ash, but came to like him once Ash protected him from an Air Slash. Sewaddle then teamed up with Pikachu to launch a Thunderbolt-Razor Leaf combo in order to scare away the Woobat. Later, Sewaddle got stuck to a wild Deerling and thrown into a river. Ash dove in to save him and the two were rescued by Burgh's Leavanny. Sewaddle was extremely grateful to Ash and allowed him to capture it. Due to the fact that Ash already had six Pokémon on his team, Sewaddle's Poké Ball sealed itself. In order to make room for Sewaddle, Ash sent Pidove to Professor Juniper's lab.
In A Connoisseur's Revenge!, Ash wanted to meet with a Pokémon Connoisseur so they could determine how compatible he and Sewaddle were. He showed it to Burgundy, a C-Class Connoisseur. She said that Sewaddle's ability was Swarm, however she believed that Ash should have a Sewaddle with Chlorophyll and told him he had to replace it. Sewaddle then proceeded to use String Shot on her in anger.
In The Lost World of Gothitelle!, Ash sent Sewaddle to Prof. Juniper's lab in exchange for Pidove.
Sewaddle returned and battled against Burgh's Dwebble in Battling for the Love of Bug-Types! and defeated it. He then went on to battle his Whirlipede and evolved into Swadloon after being hit by a powerful Solar Beam, learning Energy Ball in the process. After defeating Whirlipede, Swadloon went up against Burgh's Leavanny. As Leavanny is its evolved form, Swadloon was defeated easily.
In BW086: Rocking the Virbank Gym! (Part 1), Ash used Leavanny to battle Roxie's Koffing. Even though it was at a type-disadvantage, Leavanny managed to knock out Koffing with Energy Ball after using String Shot to spin it around. He stayed in to battle Roxie's Scolipede and got badly poisoned by Scolipede's Toxic. The battle continues in Rocking the Virbank Gym! (Part 2), where Leavanny's Energy Ball misses Scolipede and Scolipede defeated Leavanny with Sludge Bomb and Rock Tomb.
In BW108: Strong Strategy Steals the Show!, Ash used Leavanny to battle against Stephan's Sawk. Despite that Sawk's attacks did barely enough damage, Leavanny's ability Swarm had got activated boosting its Bug Type moves like String Shot and X-Scissor, but was taken down by Sawk's Karate Chop.
|String Shot +||Sewaddle and Burgh in Pinwheel Forest!|
|Razor Leaf +||Sewaddle and Burgh in Pinwheel Forest!|
|Tackle||Sewaddle and Burgh in Pinwheel Forest!|
|Bug Bite||Sewaddle and Burgh in Pinwheel Forest|
|Energy Ball +||Battling for the Love of Bug-Types!|
|X-Scissor +||Strong Strategy Steals the Show!|
|+ indicates this Pokémon used this move recently.*|
- indicates this Pokémon normally can't use this move.
- Leavanny is both Ash's fourth Starter Pokémon. -type Pokémon and his first -type that is not a
- Leavanny, as a Sewaddle, is the first of Ash's Pokémon to evolve during a Gym battle; the others were Boldore and Unfezant.
- Leavanny is Ash's only Bug-type Pokémon to have activated his ability.