Dawn is the female player character in Pokémon Diamond and Pearl as well as Pokémon Platinum. If the player chooses Lucas, the male player character, Dawn will be Professor Rowan's assistant. She helps the player by teaching them about important elements in the game, such as item shops and Pokemon Centers, how to catch Pokemon, and also gives items during the adventure. She is also part of the Pokédex project. The other blond haired rival will fight you throughout the game, but Lucas/Dawn (depending on the choice of being a boy or a girl) will only assist you politely. In the game, you will befriend others, such as the champion, and people you tag team with, like Barry, before and after you beat the game. Dawn will always be in possession of the other starter that wasn't chosen, the one which the player's starter will have an advantage against. i.e. if the player has chosen Chimchar, she will have a Turtwig in her arsenal. The reason being that if the player character is Lucas, you won't battle her at any point because she has the Starter Pokémon with a type disadvantage to his own. Similarly, if the player character is Dawn, Lucas will choose the disadvantaged starter.
However she kept her gold barrettes instead of her pink ones
In Pokemon Black and White when you first beat Cynthia she mentions Dawn, referring to her as "That Trainer who faced Giratina". She then admits she was rambling about things that didn't concern Hilbert/Hilda. (It is assumed that she could mean either Lucas or Dawn)
In Diamond, Pearl, and Platinum, Dawn has an unnamed father that works at Professor Rowan's lab.