Cynthia/Manga/PS/History/DP

Cynthia first appears in Ring Around the Roserade II. She is first seen in Eterna City, where she correctly predicts the evolutions of, , and 's Tru, Chimler, and. After laughing at Diamond and Pearl's comedy routine, Cynthia offers to give Platinum advice for her upcoming Gym battle against Gardenia. Once finished, Cynthia attacks the Team Galactic Eterna Building in order to rescue Rad Rickshaw, who was kidnapped by Team Galactic.

In Suffering Psyduck, Diamond, Pearl, and Platinum encounter Cynthia again at the. Cynthia reveals that Mr. Backlot, the owner of the mansion, asked her to investigate a that went missing. She takes the three to, where a large group of Psyduck have clustered together, creating a roadblock that prevents anyone from passing through the route. Using a SecretPotion, Cynthia cures the Psyduck of their headaches, causing them to disperse and allow travel through Route 210.

Afterward, Cynthia and the trio meet with Cynthia's grandmother, who had been forced to stay at the Café Cabin due to the roadblock preventing her from going back to Celestic Town. Cynthia decides to have her grandmother take the trio to Celestic Town while she goes off to return one of the Psyduck to its owner. Before leaving, Cynthia lends her to her grandmother for protection.

In Halting Honchkrow, Cynthia appears at the Team Galactic HQ to confront Cyrus, the leader of Team Galactic. Cynthia reveals that her intentions were to take revenge on Cyrus for attacking Celestic Town, her hometown. Despite her best efforts, Cynthia's rage leads to her commanding Garchomp to attack with, which it had yet to master. The attack heavily damages Cyrus's, but fails to actually knock it out. With Garchomp left exhausted after using the move, Cyrus bids Cynthia farewell and heads to the Spear Pillar to begin his plans.

Cynthia, realizing that Cyrus intends to summon, decides to follow after him to prevent the two Pokémon from being awakened. As she heads out of the building, Cynthia comes across Diamond, Pearl, and Platinum, who had fought Team Galactic to free from their clutches. The trio join Cynthia and travel to the Spear Pillar in order to put a stop to Cyrus's ambitions. By the time the group arrived at the Spear Pillar, Cyrus had already successfully summoned Dialga and Palkia and had them fight each other. The clash between the two Legendary Pokémon creates distortions in time and space that slowly begin to engulf the Sinnoh region. The distortions, combined with her injures, prevent Cynthia from being much help to Diamond, Pearl, and Platinum during the battle against Cyrus.

During the battle, an explosion causes Cynthia to drop a scroll containing information on Dialga and Palkia. After translating the scroll, Platinum relays the information to Diamond and Pearl, who successfully free Dialga and Palkia from Cyrus's control. Now free, Dialga and Palkia combine their powers to defeat Cyrus and stop the distortions affecting Sinnoh.