Leavanny made her debut as a Sewaddle in Pinwheel Bug Battle! She was part of a group of Sewaddle who were fighting over territory with a group of Venipede. Sewaddle encountered Lewis and the rest of the group after she had been blown away from the battle. Befriending Lewis, Sewaddle asked for his help in getting back to her friends.
When they eventually found the warring Sewaddle and Venipede, Lewis was hit with a Razor Leaf from a member of the Sewaddle army, causing this Sewaddle to angrily attack with Tackle. With both armies turning on Sewaddle, Lewis stepped in to protect her and managed to convince the two armies to live together peacefully. The other Sewaddle and Venipede apologised and Sewaddle decided to join Lewis' team.
Sewaddle took part in her first battle with Lewis in It Came From the Lake!, where she battled against a Trainer's Poliwhirl. Sewaddle was unable to make use of her type advantage due to her not knowing any Grass-type moves. Poliwhirl quickly gained the upper hand, making use of its long-range moves to keep Sewaddle - who's only damaging move was Tackle - at bay. Eventually, Sewaddle was hit by a powerful Dynamic Punch, sending her flying into a nearby lake. Underwater, Sewaddle gathered her resolve and used String Shot to pull herself out. As she came out of the water, Sewaddle evolved into Swadloon. Having learnt Razor Leaf as a result, Swadloon now battled Poliwhirl on more even grounds, finally managing to defeat it with Tackle.
Lewis used Swadloon as his first Pokémon in his Gym battle with Cheren in Lewis Gets Schooled! She faced off against Cheren's Cinccino and was easily defeated by Tail Slap.
In Insult to Injury!, Swadloon was used by Lewis to battle Benga. She fought Benga's Sawk and was defeated by Karate Chop, despite landing a few hits with Razor Leaf.
Swadloon was used in Lewis' rematch with Cheren in Re-Class is in Session! She again battled Cinccino and this time was able to use String Shot to dodge Cinccino's attacks. After a hard-fought battle, Swadloon defeated Cinccino with Tackle. She then faced off against Cheren's Watchog and was switched out for Oshawott after being hit by Watchog's Flamethrower. Swadloon was then sent out once again to face Cheren's Stoutland. Due to the harsh sunlight created by Stoutland's Sunny Day, which Cheren had used to reduce the power of Oshawott's Water moves, Swadloon was able to activate her Chlorophyll Ability, allowing her to outmaneuver Stoutland's close-range attacks and deal major damage with Tackle and Razor Leaf. Swadloon then immobolised Stoutland with String Shot and defeated it with Razor Leaf, earning Lewis his Basic Badge.
In A Plasma Civil War!, Swadloon was sent out alongside the rest of Lewis' Pokémon to battle Ghetsis and his Pokémon. She faced off against Ghetsis' Drapion and was able to avoid any major damage using Protect. However, Drapion eventually poisoned her with Cross Poison and dealt major damage with Fire Fang. Not willing to watch Unova freeze, Swadloon gathered her resolve and evolved into Leavanny. Using her newly-learnt Leaf Blade, Leavanny succeeded in knocking out Drapion. However, once Ghetsis gained control over Kyurem and commanded it to freeze Lewis and his Pokémon, she was was recalled in order to protect her.
Leavanny was Lewis' first Pokémon in his Gym battle against Marlon in Turning the Tide! She battled against Marlon's powerful Wailord, but was quickly overwhelmed by the Float Whale Pokémon's sheer size, forcing Lewis to recall it for Aipom. She was sent back out to face Jellicent, and was able to land a big hit with Leaf Blade. However, Jellicent's Cursed Body disabled Leaf Blade, putting Leavanny at a disadvantage. Eventually, Leavanny managed to defeat Jellicent with Razor Leaf. Leavanny stayed in to battle Carracosta. She was able to counter Carracosta's Rollout by wrapping it up with String Shot and then slamming it into the ground. Leavanny was then defeated by a Shell Smash-boosted Ice Beam.
In Take to the Battle Stage!, Leavanny was sent out as Lewis' first Pokémon in his Gym battle against Roxie. She faced off against Roxie's Tentacruel and was able to defeat it with Leaf Blade, although she was poisoned by Poison Jab. Leavanny was later sent out to battle Roxie's powerful Koffing, but was easily defeated by Venoshock.
Lewis used Leavanny in the first preliminary round of the Unova League in Let the Games Begin! She battled against a Trainer's Heatmor and took severe damage from Flame Burst. Leavanny then used String Shot to cover Heatmor's mouth, preventing it from using any Fire-type moves. She then hit Heatmor hard with Leaf Blade before taking it down with her newly-revealed X-Scissor.
Leavanny was Lewis' first Pokémon in his Unova League battle against Benga in Benga's Last Stand! She fought against Benga's Zoroark, but was unable to defeat it, forcing Lewis to switch her for Aipom. She later battled against Benga's Dragonite and lost easily.
Lewis used Leavanny in his battle against Hugh in At the End of the Line!, where she took on Hugh's Gothitelle. After a tough battle, Leavanny was able to defeat Gothitelle with a super-effective X-Scissor. She was then defeated by Serperior, despite a type advantage.
In A Fierce Clash of Titans!, Leavanny battled against Tunja's Metagross. She managed to strike Metagross with an X-Scissor before being defeated by a powerful Meteor Mash.
Lewis left Leavanny at Professor Juniper's Laboratory when he decided to go to Sinnoh.
As a Sewaddle, Leavanny was shown to stand up for what she believes in and sought to protect others, as seen when she attacked the Sewaddle responsible for hurting Lewis. The fact that she was willing to attack one of her own friends shows how strongly Sewaddle wishes to protect others. This particular character trait was the driving force behind her evolution into Leavanny, so that she could help protect the entire Unova region from being frozen. She is also pacifistic to an extent, believing that the territorial battle with the Venipede was pointless, but is also willing to battle if she deems it necessary. Despite her relatively small size, Sewaddle is more than capable of holding her own in battle, using her String Shot in a variety of clever ways. Due to her limited movepool, Sewaddle often had a hard time dealing with opponents with access to a larger range of attacks.
After evolving into Swadloon, she became much quieter and serious, although she still remained loyal to Lewis, hugging him after evolving. Her skills in battle have also improved and she seems to adopted a few of Lewis' personality traits, such as adamantly refusing to give up in battle.
As a Leavanny, she retained her caring attitude, as well as her drive in battles, even against much stronger opponents, such as Ghetsis' Drapion.