As a Sewaddle from his debut, he was naughty and serious, and this demonstates his attitude by attacking Pikachu in a mischievous way and showing his stubborn streak when doing naughty things towards Pikachu and later Ash who wanted to capture him. However after Ash saves him from a Pokémon attack and almost being fall into a waterfall, Sewaddle finally shows his calm and nicer side. Like Ash's Swellow, Sewaddle does not given up the fight despite its type disadvantage. Sewaddle is also very fond of Burgh the Gym Leader of Castelia City when he accompanies him at the Pinwheel Forest.
As a Swadloon, he maintains his pre-evolved personality as well as battle tactics when fighting against other Pokémon.
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 angrily uses String Shot on her face due to her judgmental remarks as well as Ash's other Pokémon because of it.
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! after Ash's Tepig is beaten down by its Rock Wrecker. Sewaddle keeps on dodging its attacks using String Shot and finally tackles Dwebble to knock him off along with its rock-house rendering it defenseless. Sewaddle attacks it with Razor Leaf followed by Tackle and defeats it with another Razor Leaf. He then went on to battle his Whirlipede taking too much damage from its Steam Roller. Sewaddle finally evolves into Swadloon after being hit by a powerful Solar Beam, learning Energy Ball in the process to deflect its Steam Roller and defeats it with Tackle. After defeating Whirlipede, Swadloon went up against Burgh's Leavanny. As Leavanny is its fully evolved form, Swadloon was defeated easily with both its Leaf Storm and Hyper Beam combination. Swadloon was soon replaced by Pikachu as Ash's final Pokémon and manages to defeat it with both Iron Tail and Electro Ball combination. Swadloon was later became happy with Pikachu and Tepig to see Ash gained the Insect Badge from Burgh.
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 Bug-type Pokémon and his first Grass-type that is not a Starter Pokémon.
- 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.