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 first appeared in Lumiose City Pursuit!.
Cosette in casual clothes
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 Summer of Discovery! to Battling Into the Hall of Fame!, 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 Foggy Pokémon Orienteering!.
She reappeared again in Performing with Fiery Charm! where she watched Serena competing in the Dendemille Town Rookie Class Pokémon Showcase along with Professor Sycamore and Sophie.
She appeared again in Cloudy Fate, Bright Future! and then in All Eyes on the Future!.
She made a brief appearance in Master Class is in Session! and Performing a Pathway to the Future!, watching Serena's performance in the Gloire City Master Class Pokémon Showcase along with Professor Sycamore and Sophie.
She appeared again in XY137, where she handed out medals to Ash, his friends, Alain, Mairin and their Pokémon for their heroic efforts against Team Flare.