Masato is the younger brother of May and the son of Norman and Caroline. At the time of the film, he is not yet ten years old and therefore does not own any Pokémon, though he carries a PokéNav to help navigate. He is an aspiring Pokémon Trainer who travels alongside his sister, Ash, and Brock, learning about Pokémon and battles through observation.

In Pokémon: Lucario and the Mystery of Mew, Masato arrives in the town of Rota during the festival honoring the legendary Aura Guardian Sir Aaron. He witnesses the tournament and later sees the Mythical Pokémon Mew teleport Pikachu and Meowth away to the Tree of Beginning. Masato is the first to notice that Pikachu has vanished and informs the group, setting the rescue journey in motion. Throughout the adventure, he acts as an observant and supportive member of the team, often offering practical advice and encouragement.

Masato's most notable moment in the film occurs during the group's tense journey with Lucario. When Lucario expresses distrust toward humans and gets into a conflict with Ash, Masato offers him a chocolate bar. Lucario accepts it and shows enjoyment, which becomes a small but meaningful step in softening his attitude. This simple act of kindness helps Lucario begin to trust the group, demonstrating Masato's empathy and gentle nature.

His personality is marked by a curious and eager attitude, though he can be headstrong and occasionally argumentative with his sister. Despite their sibling squabbles, he genuinely cares for May and supports her goals. He does not possess any combat abilities or special powers; his main contributions are his observational skills, his courage to be part of a dangerous mission, and his capacity for small, heartfelt gestures. Over the course of the film, Masato shows growth in his understanding of Pokémon bonds and the weight of history, but his core role remains that of a kind-hearted child who helps unite the group through compassion.