Yamask appeared before Team Rocket in a warehouse in Battling For The Love of Bug-Types!. Meowth suggested that they capture him to increase their strength. Jessie agreed and called out her Woobat to battle him, but James claimed he was hungry and he proceeded to feed him. Once he was full, he happily went into James's Poké Ball, the trio welcoming their newest addition with enthusiasm.
In Scare at the Litwick Mansion!, Yamask befriended a group of Litwick in a mansion that Team Rocket was temporarily using as a base. However, he was kidnapped by the Candle Pokémon in an apparent attempt to lure James, Jessie and Meowth into a trap. Yamask teamed up with Jessie's Woobat, Oshawott, Pikachu, and Dwebble to defeat the Litwick and their Lampent leader.
Sometime before An Epic Defense Force!, Yamask helped Team Rocket kidnap Meloetta, who was living in hiding, by distracting Meloetta's protecter, Ridley, with his Haze attack. In All for the Love of Meloetta!, Yamask helped uncover Meloetta while Team Rocket were pursuing it. Later in Meloetta and the Undersea Temple! he surrounded Ash and the others with Will-O-Wisp and then revealed Meloetta from its invisible state. Yamask continued to battle Axew and Pansage while Giovanni pursued Ash who had then fled with Meloetta. Then in the next episode Yamask and Woobat used their combined attacks to launch Ash of the top of the ancient temple to prevent him from interfering with Operation Tempest. When Team Rocket were finally defeated, Yamask allowed them to retreat by concealing them all with Haze.
Yamask made his final appearance in The Dream Continues!, where he attacked Ash, Professor Oak and Alexa with Will-O-Wisp. Later in the episode, he was given away to Giovanni for Team Rocket's cause.
Personality and characteristics
James's Yamask using his mask as a shield
Like many of James's Pokémon before him, Yamask is very friendly and is shown to have a great relationship with his Trainer. He is a very thankful Pokémon, as shown when he let James catch him after he was given some food. Unlike several of James's other Pokémon, such as Victreebel, Cacnea and Carnivine, Yamask doesn't show any kind of special affection to James when coming out of his Poké Ball.