Dawn Summers

Name: Dawn Summers
Age: 16
Type: The Key (mystical being)/Human
Family: Mother – Joyce Summers, Sister – Buffy Summers
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.
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.
Luckily for Dawn, Willow was able to bring Buffy back from the dead. Still, Dawn's life wasn't easy for her (being not quite real, having such a strange life, and with her sister being detached and distant after her resurrection), and she developed a thieving habit as a cry for attention.
This was eventually discovered, and Buffy has since improved her relationship with Dawn, who is now attending high school at the newly rebuilt Sunnydale High School.