From Bulbapedia, the community-driven Pokémon encyclopedia.
Sebastian (Japanese: セバスチャン Sebastian) is a minor recurring character of the Pokémon anime, who appeared in The Great Eight Fate! and Eight Ain't Enough. He is the assistant for the Sootopolis Gym, as well as Juan's butler.
He formally greeted Ash and his friends as they arrived with Juan at the Sootopolis Gym's dock. Sebastian guided Ash to his battle podium, and then served as the referee for Ash's Gym battle against Juan.
Later on, Sebastian congratulated Ash on his victory and informed him about the location of the Ever Grande Conference, which will be held in three months' time.