Reggie is a Regigigas, and a member of Serena's Pokémon Team.
Sometime after starting her journey, Serena traded a Pokémon for Reggie in a Wonder Trade. Unfortunately, Reggie refuses to follow her commands, and is mostly useless. He did, however, successfully knock out James while he was distracted by Namor.
081 The Pokéning - Part 3
087 The Pokéning - Part 9
089 The Pokéning- Bonus: Serena's Trainer Card