Some episodes later we find out the truth: A vicious hell god wants to destroy the world and to be able to do that she needs “The Key”. This is a mysterious, very powerful form of energy, protected by some monks. These monks want to keep The Key away from the god, so they transform it’s energy, make it human.
The Key now is a fifteen-year-old, a teenage girl who doesn’t know who she really is; all her memories are built by the monks, to make her and the world believe that she is an ordinary girl named Dawn Summers, whose sister happens to be the slayer.
When Buffy finds out who her sister really is she decides to protect her, no matter what. She manages to defeat the evil hell god and now Dawn is able to go on with her life as a more or less ordinary girl.
She worked with Sarah Michelle Gellar on "All My Children" before "Buffy" and they were good friends. Sarah was the one who recommended Michelle for the part of Dawn.