Dawn Summers is the younger sister to Buffy Summers in Buffy The Vampire Slayer, but she wasn't always flesh and blood human girl. She was mystical energy referred to as "the key" manifested by a group of monks into Dawn so that Buffy would do whatever necessary to protect her. As "they key," Dawn's blood can open other dimensions and so she is coveted by the demon goddess Glory in season 5. Prior to that in the show's timeline, Dawn didn't exist. She just shows up and all of a sudden Buffy has a sister.
Fans weren't exactly thrilled at this development and Dawn's character arcs over time weren't well received. From her schoolgirl crush on Xander to her general teenage angst and her on and off loathing of Buffy, Dawn's character gets worse and worse as the seasons progress.
Dawn's Teen Angst Pales In Comparison To The Troubles Of Buffy And The Gang
From the moment Dawn appears, she participates in the entire spectrum of teen angst. She writes angrily in her diary, sends evil eye looks to Buffy and her friends behind their backs, whines to Joyce about everything, and acts up in school.
Dawn's angst only gets worse as she finds out that she's different, discovers that she's the key, loses her mother, and has all kinds of big things to work through. She matures slowly, if at all, and it does her character development no favors.
Her Reckless Behavior Caused Many Dangerous Situations
Like many teens her age, Dawn believes herself to be invincible. She runs away, taunts monsters, sneaks out to hang with boys, and goes to Spike's crypt looking for scary stories. With Buffy's life being what it is, especially after Buffy and Dawn's mother Joyce dies, the gang seems to spend an awful lot of time cleaning up Dawn's mistakes, trying to find her, and rescuing her from things that could have been avoided.
Dawn Blamed Herself For The Entire Glory Problem
After discovering that she is the key, Dawn's behavior becomes increasingly troubled. She cuts herself and asks if what she sees is really blood. She burns all of her diaries. She blames herself for Tara's brain being sucked out by Glory and for Spike being tortured for information about Dawn being the key. She calls herself "a lightning rod for pain." In the end, she resigns herself to being a sacrifice but lets Buffy jump into the void instead.
The Inflated Sense of Self Importance
In Dawn's debut episode, her ominous last lines of inner monologue are all about how Buffy underestimates her. This is before she finds out that she's The Key. It goes along with the typical teenager attitude but at the same time, her inflated sense of self-importance is part of what keeps getting her into trouble. She thinks she can handle anything and doesn't understand why she's not allowed to be just like Buffy. This is an ongoing theme through the last season when she believes she could be a potential slayer, and just becomes more pronounced up to that point.
Her Shoplifting Phase
Dawn lifts things off her friends, stores around town, and even from The Magic Box, the store owned by Giles and frequented by the gang - a habit that develops over the course of the show. She takes no consideration for how important or sentimental each item might be and she doesn't steal things because she needs them. She admits later on that she even stole one of Buffy's birthday presents instead of purchasing it.
Taking Buffy Completely For Granted As A Parent
After Joyce dies, Buffy takes on the role of parent to Dawn. Before long, the school is worried and Dawn's behavior leads to the near-removal of Dawn from their household. Dawn doesn't seem to care until the dire situation is revealed.
She frequently lets Buffy know that she's not Joyce. Buffy takes a job at a fast-food restaurant and works long hours to make ends meet but Dawn complains about the food and often goes off with her friends, despite Buffy's hard work.
Almost Married The Demon Sweet
In Season 6, Xander accidentally summoned Sweet via a talisman he believes will bring more merriment and happy dancing to his upcoming wedding. Dawn, in her kleptomania phase, steals the talisman and leads the demon to believe that she was the one who summoned him. He entices her to follow him to his hell dimension as his bride and in her cloud of teen angst, she almost goes with him - or would have been forced to, if she didn't admit that she stole the talisman, and didn't originally use it.
Dawn's Dalliance With A Vengeance Demon
Halfrek, the vengeance demon, is another summoning that results from Dawn's teen angst. Her wish leads to everyone being stuck in the Summers house with her with no way to escape. She wants the world to revolve around her and hadn't seen that people still cared in their own ways without putting her in the center. It is through this wish that her collection of stolen items is found. Her big feelings are revealed and she gets exactly what she wants before the gang finds a way to escape the house.
"The First" Convinced Dawn That Buffy Wouldn't Choose Her When It Mattered
In Season 7, The First Evil returned to mess with everyone's heads. Dawn became convinced that when it really mattered, Buffy wouldn't choose her and would cast her aside. This brought all of Dawn's feelings of abandonment back to the surface. She'd worked hard up until that point to become useful as a sort of "Junior Watcher" but she wanted more than anything to be chosen as a potential slayer instead. Dawn is quick to believe this notion instead of having faith that her sister, who died for her, would always have her back.
Dawn's Perpetual Schoolgirl Crush on Xander
The memories inserted into everyone's heads when Dawn appears on the scene include her worship of Xander in a schoolgirl crush that continues throughout her teen years and into the end of the show. This crush makes Dawn hate Anya at first, resent Buffy every time she says or does something remotely degrading towards Xander, and inflates Xander's fragile ego when he comes back to Sunnydale as a townie instead of traveling the world.