Leavanny first appeared as a Sewaddle while Ash and his 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, while he kindly said hello to Iris, Pikachu, Axew and Burgh. Ash, Iris and Burgh then followed Sewaddle throughout the forest, watching him eating, as well as watching 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 scared off 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 its 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. However, 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 Connaisseuse 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, with no reason to back it up. When she said he should replace Sewaddle, he fired a String Shot at her in anger.
In Battling For The Love of Bug-Types!, Sewaddle was used against Burgh, where he fought against and defeated Burgh's Dwebble, who had previously defeated Ash's Tepig. 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.
During a battle with Iris's Emolga in BW074, Swadloon evolved into Leavanny. Immediately after evolving, Leavanny's caring nature came to the fore and he made leaf hoods for three nearby Sewaddle, after which he then made some for the Pokémon belonging to Ash and Iris. When Axew was kidnapped by Durant who, because of their bad eyesight, had mistaken the leaf hood for food, Leavanny proved instrumental in rescuing him.
As a Sewaddle, Leavanny was childish, often doing things without warning when Ash first met him, such as attacking Pikachu for no reason other than to battle. 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.
As a Swadloon, Leavanny has also been competitive since his debut, and has a strong passion for battling. He always tries 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, Swadloon is a bit more mature, although he still got a bit childish when he believed Oshawott, Tepig and Scraggy took his food in Emolga and the New Volt Switch!. However, he still has a passion for battling. Swadloon also has some sewing skills, as seen in Movie Time! Zorua in "The Legend of the Pokémon Knight"!, where he helped make costumes for Luke's movie with his Leavanny.
After evolving into Leavanny, he adopted a more caring nature and acts like a parent to the other Pokémon.
