Dawn is the female protagonist in Pokémon Pearl and Platinum, while Lucas is the male protagonist in Pokémon Diamond and Pearl.
While you don't battle Dawn in Pokémon Diamond, you do encounter her in the game! She appears in the following locations:
* Jubilife City: You'll meet her in front of the Pokémon Center after you've obtained your first Pokémon.
* Route 202: She appears again on this route, near the entrance to the Valley Windworks.
It seems you're mixing up some information about the games! Let me know if you have any other questions about Pokémon Diamond or the other games in the series. 😊