Title: Buffy the Vampire Slayer (2000)
Character: Dawn Summers

Episodes: She first appeared in the episode Buffy vs. Dracula (5.01) and her final episode was the last of the series Chosen (7.22)
Creator: Joss Whedon
Main Episodes: Real Me, Blood Ties, The Body, Forever, The Gift, All the Way, Older and Far Away, Grave, Lessons, Him, Potential, Chosen.
Also Starring: Sarah Michelle Gellar, Alyson Hannigan, James Marsters, Nick Brendon, Anthony Stewart Head, Emma Caulfield..

Summary:
Dawn was introduced in season five as the sister Buffy never had. Over season five it was discovered that she was The Key and her relationship with Buffy grew. At the beginning of season six Buffy returned from her grave and she pushed her friends and Dawn away. Dawn responded by stealing and crying out for attention and love. The year ended with Buffy telling Dawn that she wanted to show her the world. In season seven (the shows final year) Dawn helped out Buffy and the Scoobies both fighting battles and researching.
 

There is a ton of information on the series Buffy and the character Dawn. All seasons are available on DVD and I highly recommend it, not just for Michelle as Dawn, it is one of my favourite shows.

Quotes:

Dawn: Lurk much?
Spike: I wasn't lurking, I was standin' about. It's a totally different vibe.

Dawn: I like how you talk to me like I can understand things. Everyone else is being all twitchy and secretive.
Spike: They're just trying to keep you safe, I expect.
Dawn: I feel safe with you.
Spike: Take that back.

Buffy: How was school today?
Dawn: The usual, big square building filled with boredom and despair.
Buffy: Just how I remember it.

Ben: Two steaming cups of chocolate goodness courtesy of whoever I swiped it from out of the cupboard. Couldn't find any marshmallows. I'll try to steal some for next time.
Dawn: Don't like 'em anyway.
Ben: What? Is that even possible?
Dawn: Too squishy. When I was five, Buffy told me they were monkey brains...

Buffy: See...
[points to Dawn's necklace]
Buffy: You're Dawn.
Dawn: [looks down] "Or, 'umad.'"

Dawn: Drive faster.
Xander: I can't.
Dawn: I could drive faster and I can't drive.
Anya: She's right. You're like a snail. A snail who's driving a car. Now give it the lead foot. We help Buffy with that demon you sent after her.
Xander: I did not send the demon after her, he used my body to eavesdrop in on our conversation.
Anya: So now what, we have to speak in an anti-demon code?
Xander: Ood-gay idea-yay An-yay.

[to Dawn]
Buffy: You do research now? Want a cappucino and a pack of cigarettes to go with it?

Buffy: Xander must've broken the talisman.
Dawn: How do you know there was a talisman?
Buffy: There's always a talisman.

Buffy: Doesn't matter how well prepped you are or how well armed you are, you're a little girl.
Dawn: Woman.
Buffy: Little woman.
Dawn: I'm taller than you.

Dawn: I like music. I'm very into Britney Spears' early work, before she sold out. So mostly her finger painting and macaroni art.

Dawn: You guys need to really to ease up with the whole dating demons thing.
Buffy: Um, hello. I'm sorry, wasn't that you having the smooch-a-thon with teen vampire last Halloween?
Dawn: See, this is why I don't want you talking to my friends.

Dawn: I'm sure there's tons of stuff like this. You know, procedures we can use that don't involve magic spells. Just good solid detective work. And we can develop a database of tooth impressions and demon skin samples and I could wear high heels more often.
Buffy: Wow, that was so close to being empowered.

Dawn: There's blood on this. Lots. Looks like The First made another sacrifice. Or a music video.

Dawn: Do you know that ancient Sumerians do not speak English?
Buffy: They're worse than the French.

Buffy: Don't you have any real homework?
Dawn: Oh, you mean, like, schoolwork?
Buffy: Yes.
Dawn: Well, I've got a system. It's called flunking out. No, just kidding. I'm paying someone to do my work. I'm kidding. I love to see your eyeballs change color when you think I'm gonna flunk out.

Dawn: My advice to you is do exactly what everyone else does, all the time.
Willow: Got it.
Dawn: Do what everyone else does, wear what everyone else wears, say what everyone else says.
Willow: Okay.
Dawn: People may say something to you you don't understand, just don't be afraid to keep your mouth shut and pretend like you know what they're saying.
Willow: You know, Dawn, I've been to college before.
Dawn: People may say something like, "My protein window closes in an hour." Just... nod and smile. "Mm-hmm." Turns out it has something to do with fitness.

Gallery:
All the Buffy caps, images etc are together here.

Outside Links:
IMDB
Much Ado About BtVS