She stays in the Pokemon League.
The Pokemon Champion is Cynthia
Cynthia
Cynthia.
Cynthia is not in a contest so who told you she was in one?
The champion in the Elite Four.
You have to get the SecretPotion from Cynthia.
Beat the Champion Cynthia
Cynthia's grandmother is in Celestic Town, on the other side of Mt. Coronent.
Her full name is Cynthia Roran Kale
In Pokemon Diamond (and Pearl), Cynthia's first Pokemon is a Spiritomb. Her other 5 Pokemon are Garchomp, Gastrodon, Roserade, Milotic, and Lucario.
Cynthia does not give you an egg in Pokémon Diamond or Pearl, she gives an egg containing a Togepi in Pokémon Platinum after you have beaten Team Galactic in Eterna City
no he is not. the champion is Cynthia not your rival