From Bulbapedia, the community-driven Pokémon encyclopedia.
Cosette (Japanese: コゼット Cosette) is a recurring character in the Pokémon anime. She is one of Professor Sycamore's assistants who 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.
She appeared briefly in Mega-Mega Meowth Madness!, clearing up the rubble Team Rocket had caused while kidnapping Professor Sycamore. She reappeared from XY039 to XY042, as 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 XY041.