Yes. She will be your partner when you battle multimode.
There is no Battle Frontier in Pokemon Diamond and Pearl. Only the Battle Tower.
after you battle 7 trainers in the battle tower you will get 3bp
The Battle Tower
in fight area
battle tower
in the battle tower
You have to win 100 battles in the battle tower
At the fight area (after you beat the Pokemon league)
you cannot ever beat the battle tower. but if you have completed the sinnoh pokedex then the battle tower will help you complete it. you can battle at the battle tower as long as you want.
The Pokemon you see do not go in your pokedex.
you can get it by exchanging battle points at the battle tower. Enjoy =]
In the Battle Tower, for 48 Battle Points.