This Darmanitan, also known as the Red Flash Darmanitan, is a fire-type Pokémon owned by Bianca's father.
When Ash tried to prove Bianca's father, who didn't want Bianca out to be on her journey, taking her home was not fair, Bianca's father had a battle with Ash. Before the battle, they made the bet so if Ash were to lose, Ash would have to go back to Pallet Town. He sent out Darmanitan against Ash's Oshawott. Despite the disadvantage, Darmanitan used many fire-type moves to overpower Oshawott and managed to defeat him.