Swadloon first appeared as a Sewaddle while Ash and friends were in Pinwheel Forest. After attacking Pikachu, 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.
Despite showing antipathy to Ash early on, Sewaddle started to change his mind about Ash when he jumped in front of Sewaddle and took an Air Slash attack from two Woobat to protect him. Sewaddle and Pikachu then used a combo of Razor Leaf and Thunderbolt to scare the two Woobat away. Afterwards Sewaddle got stuck on a Deerling and got 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 to Ash and happily allowed him 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.
Sewaddle sleeping
In BW019, 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 Connoisseur by the name of Cabernet. 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. When she said he had to replace Sewaddle, he fired a String Shot at her in anger.
In BW021, Ash took back Pidove from storage in exchange for Sewaddle.
In BW025, he was taken out of storage for use against Burgh, where he fought against and defeated Burgh's Dwebble. Later, while battling his Whirlipede, Sewaddle evolved into Swadloon, while learning Energy Ball in the process. However, he lost to Burgh's Leavanny after being hit by a Leaf Storm and a Hyper Beam.
