Pre A &A: Buffy the Vampire SlayerEditDawn is first introduced as Buffy's little sister at the end of Buffy season five, despite Buffy having 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.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. 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. Dawn then suffers more pain when her mother dies unexpectedly from a brain aneurysm which leads to Dawn resorting to black magic to bring her back from the grave, an action she immediately reverses upon realizing the consequences of her actions. It is eventually revealed 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.
Season six sees Dawn struggle with abandonment issues, as well as her escalating kleptomania. Having been devastated by her sister's death, Dawn is overjoyed when Willow casts a spell to bring her back to life. She later experiences her first kiss with a vampire named Justin, whom she is reluctantly forced to stake (her first vampire kill) when he tries to turn her. Dawn's isolation from the other characters reaches its apex when she inadvertently makes a wish to the vengeance demon Halfrek which results in them becoming temporarily trapped inside the Summers house. Everyone is eventually freed when Halfrek reverses the spell, and Dawn's kleptomania and feelings of neglect by her friends and family are exposed. While helping Buffy battle demons in the season finale, Dawn proves herself to be capable in a fight, finally earning her sister's respect. In season seven, Dawn becomes more grown-up and a fully-fledged member of the "Scooby Gang". Falling victim to a love spell in the episode "Him", she displays dangerous behavior such as attacking people and trying to commit suicide to prove her "love" for classmate RJ Brooks. While home alone one night, Dawn is forced to perform a solo exorcism to protect what she believes to be her mother from a demon, though it is revealed to be The First Evil attempting to cause Dawn to doubt her bond with Buffy. After wrongly believing herself to be Potential Slayer, Xander explains to Dawn that being normal is perhaps the hardest burden of all, as nobody understands the pain of being overlooked–as he has been in Dawn's shoes. When Buffy tricks her into leaving town before the impending apocalypse, Dawn adamantly returns to fight against the First, during which she battles alongside Xander and once again proves her worth by killing several Ubervamps and survives.
In the comic book continuation to the television series, it is revealed that Buffy and Dawn's relationship has been strained and conflicted since the events of "Chosen." Sometime before the series begins, Dawn becomes a giant which Buffy believes is the result of losing her virginity to a "thricewise demon" named Kenny. Later in the series however, Dawn confides in Xander she in fact slept with Kenny's roommate and her becoming a giant was due to a spell cast by Kenny. As a giant, Dawn's personal issues are forced to take a backburner while she finds interaction between friends and family more strained. In battles however, her gigantism shows some utility; she is able to assist fighting Amy in "The Long Way Home", and rampages through Tokyo to distract their enemies in "Wolves at the Gate".
In "Time Of Your Life, Part One" she shrinks to normal size again, but then turns into a centaur. Xander surmises she will likely experience a third such transformation before she can return to normal. In "Living Doll", the twenty-fifth issue of the series, Dawn undergoes her final transformation into a living doll and is kidnapped by a disturbed dollmaker for her 'protection'. Buffy, Willow, Andrew and the Slayers manage to locate Kenny and confront him about the curse, but he flees, only to show up later alongside Buffy to rescue Dawn. After she apologizes to him, the spell is instantly broken and she becomes a regular girl again. The two have a heart-to-heart, where she explains she wasn't ready for sex with Kenny because of how intensely she liked him, before parting ways again. At the end of the story, she and Buffy mend their underlying issues, particularly that Buffy is now responsible for hundreds of Slayers and cannot devote her attention to Dawn.