Leavanny first appeared as a Sewaddle in BW018, 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 resuced 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 BW019, 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.