From Bulbapedia, the community-driven Pokémon encyclopedia.
Cosette (Japanese: コゼット Cosette) is a recurring character who appeared in the Pokémon anime. She is one of Professor Sycamore's assistants.
Cosette in casual clothes
Cosette first appeared in Lumiose City Pursuit!. While Ash and his friends were talking to Professor Sycamore, Cosette came by. Professor Sycamore had her look after the lab's Garchomp while he took the group outside. Cosette then researched Garchomp using a machine to learn about Mega Evolution. When Team Rocket walked in, disguised as scientists, Cosette walked in with Garchomp as she and Sophie did not recall having them invited and would check with Professor Sycamore. However, before she did so, Team Rocket threw a mechanical collar at Garchomp even though it was meant for Froakie. As Garchomp went on a rampage, Cosette ran outside and warned Professor Sycamore about what was going on. Cosette stayed behind in the lab while Ash and his friends went after Garchomp.
In Mega-Mega Meowth Madness!, Cosette cleared up the rubble Team Rocket had caused while kidnapping Professor Sycamore. From Summer of Discovery! to Battling Into the Hall of Fame!, Cosette was part of the staff at the Professor's Summer Camp. Amongst her activities was refereeing a battle between Ash and Tierno and judging the teams in the various activities, and guiding the teams at a stamp table during the PokéEnteering competition in Foggy Pokémon Orienteering!.
In Performing with Fiery Charm!, Cosette watched Serena competing in the Dendemille Town Rookie Class Pokémon Showcase alongside Professor Sycamore and Sophie.
She briefly appeared again in Cloudy Fate, Bright Future! and All Eyes on the Future!.
In Master Class is in Session! and Performing a Pathway to the Future!, Cosette was watching Serena's performance in the Gloire City Master Class Pokémon Showcase alongside Professor Sycamore and Sophie.
In Battling With a Clean Slate!, Cosette handed out medals to Ash, his friends, Alain, Mairin, and their Pokémon for their heroic efforts against Team Flare.