Masato, Haruka's younger brother, travels with Satoshi and Takeshi to Dr. Yung's Mirage Mansion, arriving after receiving an invitation. During Dr. Yung's demonstration of the Mirage System, Masato observes the battle between Misty and a Mirage Aggron. He notices the Aggron using attacks beyond its normal capabilities – Thunder Wave and Bullet Seed – prompting Dr. Yung to explain that Mirage Pokémon can use any move due to programmable data.

When the Mirage Master attacks, kidnapping Dr. Yung and Professor Oak, Masato stays outside the castle grounds with the group. He witnesses Pikachu's distress signal during Satoshi's capture. Alongside Haruka, he waits outside while Satoshi and Misty infiltrate the mansion, agreeing to seek help if needed. Later, he observes the intense lightning blast from Pikachu's amplified Thunderbolt attack. This alerts both Masato and Haruka to the escalating danger inside, prompting Haruka to send her Squirtle to assist Misty.