In 2000, a Czech Republic-based brethren known as the Monks of Dagon have possession of The Key, a mystical ball of energy that can open a portal to all dimensions. Once opened, it would create chaos on Earth as hell dimensions flood into this world until the energy is used up and the portal closed. The monks wanted to put this dangerous power to good use, but forces of darkness tracked them down. In an act of desperation, three surviving monks used powerful magics and used part of Buffy to forge it into human form - a fourteen-year-old girl - and sent it to the Slayer as a sister to protect it from the unstable Glorificus. Also known as Glory, the hell-god was bent on returning to a hell dimension from which she had been exiled for being too vicious.

Dawn first appeared at the very end of the episode "Buffy vs. Dracula ", when Buffy walked into her bedroom (preparing for a date at the movies), she finds Dawn looking through her stuff. Buffy's mother then suggested; "Buffy, if you're going to the movies, why don't you take your sister?" and both Buffy and Dawn shouted "MOM!". She became a series regular in the last three seasons, appearing in all 66 episodes. Her character, Dawn Summers, was supposedly born to Hank and Joyce Summers in 1986 in Los Angeles. Dawn's immediate family were unaware of her mystical origin for a while as they had been imbued with artificial memories of her as Buffy's younger sister. Only those "outside reality," such as mental patients, see Dawn's true nature.

Her brain tumor let Joyce see that Dawn was different, and Dawn struggled to accept her existence and others' love upon learning the truth. After the shocking death of Buffy, she struggled with kleptomania, overcoming her problems with the help of Rupert Giles and friends. By becoming more involved with Buffy's slaying, Dawn grew less dependent on those around her. She began to take an active role in helping the Scooby Gang battle its foes.