Yamask appeared before Team Rocket in a warehouse in Battling For The Love of Bug-Types!. Meowth suggested that they capture it to increase their strength. Jessie agreed and called out her Woobat to battle it, but James claimed it was hungry and he proceeded to feed it. Once it was full, it happily went into James's Poké Ball, causing the trio to cheer.
Like many of James's Pokémon before it, Yamask is very friendly and is shown to already have a great relationship with its Trainer. It is a very thankful Pokémon as shown when it willingly entered James's Poké Ball after it was given a little food. Also, unlike most of James's other Pokémon, such as his Victreebel, Cacnea and Carnivine, Yamask doesn't show any kind of special affection to him when coming out of its Poké Ball.
As seen in BW037, it can use its mask to deflect attacks similar to how Ash's Oshawott uses his scalchop. It can also, like Jessie's Seviper, use Haze to shroud Team Rocket in a dark cloud of smoke to allow for a quick getaway.
A † shows that the move was used recently, unless all moves fit this case or there are fewer than five known moves.
Trivia
Yamask is the first Ghost-type Pokémon to be officially owned by a main character.
This is the first Pokémon James has added to his party since Carnivine in Two Degrees of Separation!, 212 episodes before. Since Carnivine was actually obtained when James was still a child, this is his first capture in 259 episodes.
Related articles
For more information on this Pokémon's species, see Yamask.