Dawn Summers isn't really anyone's love child. Sometimes siblings just appear out of nowhere and come pre-packaged with bogus memories. When the youngest Summers arrived in Sunnydale, she was your typical teen, an obnoxious little monster who spent more time getting under Buffy's skin than actually helping out.
Several months after Dawn's arrival, Buffy and the Scooby Gang discovered her true identity: she's really The Key, an epical powerful energy source transformed into human appearance by benevolent monks. As an average bratty teen, Dawn didn't much like finding out that she was actually a cosmic force. As a matter of fact, she ran away from home. Buffy managed to convince her that they are family regardless of her origin, but their happiness was cut short with the unexpected death of their mother, Joyce.
Shortly thereafter, a nasty and potent Hellgod, Glory, arrived to find the Key and try to use her to open a portal that would release hell on earth. Despite knowing that her sister would be left entirely alone, Buffy sacrificed herself in Dawn's place to prevent the portal from opening. Unable to accept the death of her friend, Willow used her rapidly increasing Wiccan powers to raise her from her grave and bring her back to Sunnydale.
Buffy's return gave Dawn her family back, but her sister's remoteness made Dawn feel neglected and unloved. She began shoplifting, much to the gang's dismay. Even her relationship with Willow was altered forever when the witch took the teen on a wild, magic-addled ride that ended with a car wreck and a broken wrist for Dawn.
A turning point came when Buffy fought the demonic skeletons brought to life by Willow's rage over Tara's death. Dawn began to fight alongside her Slayer sister, and Buffy was amazed at the strength and power the girl suddenly displayed. Together they were victorious, and Buffy gained a new respect for her younger sister, much to Dawn's relief. But now, Dawn must face one of the most hazardous experiences the universe has to offer: high school. Lucky for her she's got the Slayer on her side.