Swadloon first appeared as a Sewaddle while Ash and 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. Ash, Iris, and Burgh then followed Sewaddle throughout the forest, watching he was eating as well as 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 defeated 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 it's 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, but 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 Connoisseur 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. When she said he should replace Sewaddle, he fired a String Shot at her in anger.
In Battling For The Love of Bug-Types!, 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.
As a Sewaddle, Swadloon was childish, often doing things without warning when Ash first met him. 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.
Swadloon has also been competitive since his debut, and has a strong passion for battling. He always tried his hardest, even when he was at a strong disadvantage during his battle against Burgh's Whirlipede where he still refused to give up and eventually evolved. Since he has evolved, not much has changed.