Dawn Summers (referred to often as "Dawnie" by friends) is {{Buffy}}'s kid sister, which was a bit of a mild surprise to fans of the series prior to SeasonFive, because for the first four years of the series, Buffy was an only child. Beginning with the fifth season premiere BeforeDawn? in which Buffy met {{Dracula}}, upon her return from that adventure Dawn just unceremoniously appeared in Buffy's bedroom, and Buffy reacted as if Dawnie had always been around. So did everyone else.
Later that year the {{Scoobies}} discovered Dawn had been {{magic}}ally created by {{monk}}s. Stay with me here. There was this hell god named {{Glory}} Glorificus? who wanted this cute little green glowing orby thing. So the monks had to hide it. This cute green glowing orby thing was actually TheKey? which could unlock the barrier between Buffy's reality and the place from which Glory had been banished. Glory wanted to get back home, but opening the portal between realities would destroy Buffy's reality, not that Glory cared much. So the monks had to hide the Key. They did so by taking the little green glowing orby thing which was all energy, and convert it into living matter. A girl. They also used Buffy's DNA as a sort of blueprint, which meant that Dawn wasn't just Buffy's kid sister. Technically Dawn was a mystical clone of Buffy. Their flesh and blood was figuratively if not literally the same. The monks did this hoping that in so doing, TheSlayer would be compelled to protect TheKey? with her life. They were right.
Dawn is for all intents and purposes a normal human girl, although it has been established that a particularly powerful {{witch}} could revert Dawn back into her original energy component. It is not known if Dawn would retain any memories of her human form if ever she were dematerialized. It's not known if she has any special abilities due to her unique origins. She is an easily excitable young woman, whose illusion is her reality. Though literally created in the fifth year of the tv series, the characters of BuffyTheVampireSlayer believe her to be approximately four years Buffy's junior. Growing at the regular rate of any human, Dawn is almost a foot taller than her sister, and looks nothing like Buffy, even though their DNA is identical. Dawn's first kiss was with a vampire that she was forced to slay soon after. She's gone through extensive training of vampire slayage with her sister as a teacher, has had a crush on both XanderHarris and {{Spike}}, feels a very close (but of course platonic) friendship to {{Tara}} and {{Willow}} and enjoys microwaving a marshmallow cuz it blows up real big. Oh. And she's into wearing high heels, as if she's not tall enough already.