From Bulbapedia, the community-driven Pokémon encyclopedia.
Joshua (Japanese: ジョシュウ Joshū) is Professor Birch's assistant.
He first appeared in Get the Show on the Road. He was watching the lab while the Professor and Ash went to go look for Pikachu. When May arrived to receive her first Pokémon, Joshua informed her that the Professor was busy. May then went off to look for him.
Joshua also appeared in A Six Pack Attack, where he was taking care of the Hoenn starters for Professor Birch.