how does buffy come back in season 6
Buffy the Vampire Slayer (season 6) At the beginning of season six Willow resurrects Buffy with a powerful spell. Although her friends believe that they have taken her out of Hell, Buffy had actually been in Heaven since her death (a fact that is revealed to her friends later in the season). She is therefore traumatized by her resurrection and this causes her to fall into a deep depression for most of the season.
Dawn Summers After years of foreshadowing, Dawn was introduced at the start of the fifth season as part of a large in-story retcon: Characters accepted Dawn's presence as if she had always been there, and as if Buffy always had a sister, with only the audience aware that this was not the case. As the series went on, the significance of Dawn's arrival is revealed to the series's other characters, and they come to understand that she has not always been Buffy's sister, or indeed a sentient being; Dawn had originally been the mystical "key" to unlocking dimensions and was made into Buffy's sister so the Slayer would protect her. Dawn is, however, a real girl, Buffy's biological sister, and has real memories of her fictional childhood. She struggles in a very human way when she discovers the truth about her origins and later endures problems with self-harm and kleptomania. The show's sixth and seventh seasons, as well as its canonical comic book continuation, follow Dawn's journey toward adulthood.