Doppel is a Ditto, and a member of Serena's Pokémon Team.
During her training session with Red, Serena caught Doppel, with help from Foxy. Doppel then demonstrated his shape-shifting abilities, transforming into a ball, a cake, a parasol, and a human. He later disguised himself as Jessie in order to distract James so the rest of Serena's team could defeat him and his Pokémon.
083 The Pokéning - Part 5
084 The Pokéning - Part 6
085 The Pokéning - Part 7
087 The Pokéning - Part 9
089 The Pokéning- Bonus: Serena's Trainer Card