Dawn is actually a spiritual entity called "The Key" who was sent to the Slayer in human form so that the Slayer would protect her. But Dawn thinks she's just Buffy's little sister, and really she practically is just a normal kid. Before Buffy's discovery of Dawn's purpose and their mother's illness, Buffy and Dawn fought constantly. However Buffy softened torwards Dawn once she realised how important the kid was to the world without knowing it. Joyce's brain tumour also improved their relationship, and it made Joyce come to the realisation that while Dawn wasn't really hers, she should still love and take care of her. Dawn is somewhat klutsy and often gets in the way, which is probably why Xander relates to her. Those two get along really well, as most of the Slayerettes do with Dawn. So basically, she's a popular kid. She pretty much flipped when she discovered that she wasn't always human, but she's had a lot more to deal with lately--what with Joyce's death and now Buffy's sarcficing herself for her.