BtVS Season 5
Dawn Summers
Name: Dawn Summers
Age: 16
Type: The Key (mystical being)/Human
Family: mother Joyce Summers, father - Hank Summers, sister Buffy Summers
Powers: Dawn is a human being who, as the mentally unstable can see, is really a green ball of energy. With the right ritual, her blood opens a dimensional portal to all universes, bringing chaos and destruction to Earth within minutes. It's never made entirely clear if Dawn is actually human or still the transfigured key, though many characters repeatedly state that Dawn was the Key after Season Five, and Dawn herself stated that she is no longer the Key, and even if she still is, she no longer opens anything.
History: After protecting The Key for centuries, the monks of the order of Dagon were forced to hide it from Glory, a hell god, by turning it into a human girl. They placed Dawn with the Slayer, disguised as a younger sister, and implanted memories in everyone in Buffy and Dawn's lives that made it seem as if Dawn had always been a part of Buffy's family.
It is unclear how the implanted memories are activated or if people outside of the Summers' lives know about Dawn. The insane and those "outside of reality," such as clairvoyants, are able to see that she is not real, and Buffy was able to identify her as part of a spell by going into a deep trance. It is also fairly unclear exactly how Dawn's presence altered past events before Season Five, though we do know that she always believed there was more than one of Ted (I Was Made to Love You), that she played chess with Willow (Real Me) and that Faith knew her before Dirty Girls and that Dawn held a grudge against her. This grudge could stem from This Year's Girl where by Faith kidnapped Joyce and possibly, in light of the altered reality, Dawn as well. Also, she and Angel apparently know each other; in "Family", when Buffy is lecturing Dawn on not falling in love with vampires (particularly her then-crush Spike), Dawn sarcastically replies, "Right, that's why you were never with Angel for three years". (Buffy is probably surprised by those words.)
Joyce seemed to recognize that Dawn was not truly her daughter at certain moments when she was being affected by her brain tumour. Dawn was initially unaware of her true nature, but learnt the truth, much to her distress, when she broke into the Magic Box and read Giles' diary.
Glory's aim in this world was to find the Key it turned out the Key could break down all the dimensional barriers, and Glory intended to use Dawn in a ritual to get back to her own dimension. This ritual would result in Dawn's death, and the unleashing of countless demon dimensions on earth. Buffy wasn't able to stop Glory from starting the ritual, but by sacrificing her own life she was able to stop it in time to save Dawn and the world.