From Bulbapedia, the community-driven Pokémon encyclopedia.
Thomas (Japanese: タクミ Takumi) is a character of the day who appeared in Teaching The Student Teacher!.
Thomas attended Johanna's "My Very First Pokémon Coordinator Class" to learn about the basics of being a Coordinator. He was initially rude with Ash and his friends, and he in particular bullied Dawn's Piplup on several occasions, including drawing on its face with a marker. However, Brock later explained that Thomas could not express the fact he liked Piplup properly, and in frustration, he resorted to teasing to get his attention.
He appeared slightly more interested in Dawn's Coordinator displays, though he criticized her "Ice Chandelier" combination failed. Despite the harshness of his criticism, it motivated Dawn to perfect the Contest combination by applying Buneary and its Ice Beam instead. During the celebration, Thomas and Piplup hugged without realizing it and laughed it off. The two later became friends and played in the nearby water by the end of the episode. Dawn thanked Thomas for his input, while he wished her good luck and apologized to Piplup for teasing him earlier.