There are 4 Heroes in the game: Batman, Robin, Batgirl and Nightwing.
Batman and Robin are unlocked from the start.
There are two ways to unlock Batgirl and Nightwing, the first way is to complete the entire Hero story, once you have done this you can purchase the 2 characters and the achievement will be yours.
Alternatively you can use cheat codes, which do not effect your achievements.
From the Batcave/Arkham Asylum computer hub enter the following codes and the achievement will pop when you exit the computer:
Batgirl - JKR331
Nightwing - MVY759
Here's a video demonstration