Dawn is the key, the key is green energy turned into human form by monks to hide it from Glory and sent to Buffy as her sister to ensure that she will protect it with her life. The implant memories of Dawn into the minds of everyone so that they believe that Dawn is really Buffy's little sister. Other than this, Dawn is a normal 14 year old girl with a crush on Xander, and later, a crush on Spike.
The only people that can tell that Dawn is other than human are those outside of reality, including the insane, which are growing in numbers with the arrival of Glory in Sunnydale. Glory needs the key to return to her own dimension, unfortunately, a side effect of opening her dimension is the opening of all other dimensions.
In an effort to see who or what is hurting her mother, Buffy goes into a trance and, instead of finding anything wrong with her mum, she learns that Dawn is not real. Effected by her brain tumour, Joyce also learns the truth about Dawn.
Dawn is unaware of her true nature until she overhears the gang talking and goes off to the magic shop to learn the truth for herself.
Tara's madness, caused by Glory, soon gives Dawn away to Glory and she captures Dawn and starts the ritual to break down the barriers to all dimensions. Once started the only way to stop the ritual is to stop the blood flowing, i.e. to kill Dawn. Realising that Dawn was created from Buffy and that her blood is the same as Dawn's Buffy kills herself thus saving Dawn and the world.
No longer a key to anything much, Dawn starts researching and fast becomes a valued member of the scoobies. She is delighted when her sister is brought back from the dead, but soon starts to feel left out and takes up stealing as a way of drawing attention to herself. When Willow goes evil Dawn eventually gets to fight side by side with her sister and shows over and over again that she has plenty of the Summers' pluck.
With the opening of new Sunnydale High School comes the old problems, demons, zombies, vampires, not to mention The First Evil beant on opening the Hellmouth and swallowing the world.
With the coming apocolypse its all hands on deck and Dawn finds herself becoming more involved with the scoobies and holds her own in the final battle.
After the destruction of Sunnydale, Dawn moves to Italy with Buffy where she continues her schooling.
