A New Dawn
At the beginning of Season 5, you could almost hear the collective gasp of fans everywhere as Buffy's seemingly-always-there, super annoying kid sister Dawn shows up (Episode 1, Buffy vs. Dracula.) What was happening? Who was she? Buffy has a sister? Is she real? Waiting for the next episode to start was like torture at the hands of the Master. Once we delved in to the season, many of us began disliking Dawn. (not the actress, Michelle Trachtenberg, who so lovingly played Harriet the Spy some years earlier; she's adorable. It's the character we are talking about here.)
Dawn whined and complained, didn't seem to fit in with the other members of the Scooby gang, and while providing some comic relief and of course, adherence to the ultimate Glory story line, she seemed to be unnecessary – what some people would call jumping the shark. While I agree that Dawn isn't the most beloved Buffy character, I think there are some valid reasons she exists.
No One Has an Older Sister Who is the Slayer
Having a little sister forces Buffy, who has never lived a normal existence, into some semblance of normalcy. Not only does she have to contend with an annoying kid sister hanging around, but she has someone she has to constantly look out for who can't look out for themselves. That puts Buffy into an entirely new role: caretaker. Yes, Buffy often saves the lives of those around her, but a helpless young person? Pretty new, and we see Buffy in a more mature light than she's ever been cast, which is crucial for plot development as Buffy's responsibilities increase towards the end of the series.
"I Don't Want to Protect You from the World; I Want to Show it to You"
Having Dawn around makes Buffy ultimately responsible, and I'm wondering if one of the reasons Dawn came along during the same season that Joyce passes away (Episode 16, The Body) is so Buffy wouldn't completely lose her way. It would have been easy for Buffy
to lose her mother and succumb to darkness herself, losing hope, becoming depressed, but with Dawn to take care of, Buffy doesn't have that option; she has to be the strong one, the adult, when before, that role was left to Giles, the only actual grown up. It forces her to stay hopeful, stay on course, and that's a good thing. She also comes to realize her own importance, in Sunnydale, in the universe, and knowing it's up to her to save the world is a pretty big cross to bear, but this season, there are many turning points, and Dawn works as a positive plot device in these situations, and a metaphor for Buffy's ultimate transformation; into a grown up.
"Dawn's in Trouble. Must be Tuesday"
One of the coolest things about Dawn is her ability to relate to other characters in the Scooby gang who also don't have special powers, like Tara (who forms a close friendship with Dawn) and Xander. It allows Dawn to find some place to fit in, and gives us another character that's more normal (well, as normal as a key that opens a dimensional portal that can destroy the universe can be, that is) and grounded in the real world and its real issues (boys, homework, curfew, etc.) making Buffy even more of a teen show that it is, by definition. And though Dawn ends up getting herself into trouble time and time again (even getting kidnapped by the devil in Once More with Feeling) she serves to bring us back to the reality of being a real teenage girl, and, well, it truly is annoying! Love her or hate her, Dawn plays a pivotal role in the Buffyverse, and hey, at least we didn't have to hear her finish her sad lament – we may have a different opinion altogether if that sad song had continued!