where did buffy's sister come from in season 5
Dawn Summers Dawn is first introduced as Buffy's (Sarah Michelle Gellar) younger sister at the end of Buffy season five premiere "Buffy vs. Dracula", though Buffy had been previously established as an only child. Initially, the mystery of Dawn's sudden existence is not acknowledged in the series, with the other characters accepting her as a part of the status quo. Four episodes later Buffy discovers Dawn is in fact a mystical object known as The Key; a group of monks transformed The Key into human form and sent it to the Slayer for protection from the villainous Glory (Clare Kramer). The memories of Buffy and her associates were altered, along with relevant records, so that they believed her to have always existed as Buffy's sister. She is shown to look up greatly to Willow and Tara and has a crush on Xander. She also is friendly with Spike, who is as protective of her as Buffy herself. When Dawn learns of her origin, she resorts to self-harm and runs away from home, until Buffy assures her they are real sisters no matter what, securing it with a blood oath. Her relationship with Buffy, having been portrayed with a typical sister dynamic in the first half of the season, changes when she discovers what she is. The two become closer as Dawn becomes Buffy's sole focus once she drops out from college to protect her, going as far as to warn her friends she is prepared to kill anyone who attempts to go near Dawn in the finale. Dawn suffers more pain when her mother (Kristine Sutherland) dies unexpectedly from a brain aneurysm, which leads to Dawn resorting to black magic to try to bring her back from the grave, an action she immediately reverses upon realizing the consequences of her actions. It is eventually revealed that Dawn's purpose as The Key is to open portals to alternate dimensions, a power the hell-god Glory wishes to exploit to return home. When Glory successfully uses Dawn's blood to break down the dimensional barriers, Buffy sacrifices her own life, realizing that their blood is now the same, to end the apocalypse and save Dawn. Buffy's sacrifice also neutralizes the power of The Key, giving Dawn the opportunity of a normal life.