Dawn Summers

Warning: Wikipedia contains spoilers.

Dawn is the younger sister of the title character Buffy Summers. She first appeared in the series' fifth-season episode Buffy vs. Dracula, and would continue to appear as a regular cast member throughout the following seasons. Her introduction in the series came as something of a surprise, since prior seasons showed no indication that Buffy had a younger sister. During the fifth season's story arc, it was revealed that Dawn was in fact a 'key'; that her true nature was a mystical energy that possessed the capability to 'unlock the doors between dimensions.' Dawn's identity had been concealed, and her human form entrusted to Buffy, by a group of monks who intended to hide the 'key' from the evil goddess, Glory.

Dawn, and those around her, were unaware of her true nature for quite some time. She had been forged into human form with a complete recollection of her false life as Buffy's sister, and those surrounding Dawn had also been imbued with artificial memories, making Dawn's eventual discovery of the facts about her existence difficult to come to terms with, and causing a crisis of identity.