BioCynthia is the Champion of the Sinnoh region, originating from Celestic Town. She is also a researcher of Pokémon mythologies and folklore. She travels across the Sinnoh region, as well as other regions, accepting challenges to Pokémon battles and studying sites related to Pokémon mythologies.
Ash, Brock, and Dawn first meet Cynthia as she is studying stone tablets at Amity Square Ruins. Cynthia instructs Ash and Paul and observes their rivalry, with Chimchar being the key that binds their fates together. As Cynthia recites from the stone tablets, "When every life meets another life, something will be born." Cynthia and Ash, as well as his friends and Paul, encounter each other throughout the Diamond & Pearl series from then on. One such instance is Cynthia's intervention to stop Cyrus of Team Galactic's plot to control of Dialga and Palkia. She appears again during the Sinnoh League Tournament, in which Cynthia is a spectator to the matches that take place.
Cynthia meets Ash again, as well as Cilan and Iris, in the Unova region, during Best Wishes 2. They travel together to Cynthia's villa, while encountering Meloetta on the way. Iris challenges Cynthia to a battle as she hopes to become a Dragon Master and raise her Axew to be as strong as Cynthia's Garchomp. Cynthia is expected to have an exhibition match at the Pokémon World Tournament Junior Cup.
Cynthia's team throughout the series consists of Garchomp, Gastrodon, and Glaceon. Garchomp is her strongest Pokémon, and most frequently called out. Garchomp helped triumph over Team Galactic and defeated multiple challengers, such as: Paul, Aaron, Flint, and Iris. Cynthia's Gastrodon has also helped in several battles, particularly against Team Galactic and against Aaron in a Championship match. Her Glaceon is called out to help heal the sickly Meloetta by creating ice.
Pokémon World Tournament Junior CupCaitlin battled against Cynthia in a 10 minute exhibition match during the opening of the Pokémon World Tournament Junior Cup at the Kagome Town stadium. The match ended in a draw though but Cynthia said that she was able to sense Caitlin growth in the battle. Caitlin was very grateful for the opportunity to battle against Cynthia and the two of them hoped to be able to battle again sometime in the future.
Cynthia's Gastrodon was first seen battling against Team Galaxy at the Celestic Ruins.