The show's titular protagonist, Buffy, is "The Slayer", one in a long line of young girls chosen by fate to battle evil forces in the form of vampires and demons.
This calling mystically endows her with a limited degree of clairvoyance, usually in the form of prophetic dreams, as well as dramatically increased physical strength, endurance, agility, intuition, and speed and ease of healing.
In another departure from the usual conventions of television, Xander is notable for being an insecure and subordinate male in a world dominated by powerful females.
In the earlier seasons, Giles researched the supernatural creatures that Buffy must face, offered insights into their origins and advice on how to kill them.
Throughout the series, he became a father-figure to Buffy, Willow, Xander, and the others, giving them advice not only on the supernatural world, but on life issues as well.
Portrayed as taciturn and unflappable, the contrast between his outward coolness and his violent animal episodes is an example of the show's efforts to subvert usual character expectations, as well as to display double-personalities (like Angel/Angelus).
As an unusual side-note, the actor Seth Green went on to co-design and supervise the creation of the acclaimed line of Buffy the Vampire Slayer action figures.
William "Spike" Pratt is a vampire character whose role varies dramatically through the course of the series, ranging from a major villain to "love's bitch", to the sarcastic comic relief, to Buffy's romantic interest in a relationship that grows from miserable lust to a friendship, and eventually to a self-sacrificing hero, dying as a Champion at the Hellmouth.
Her story is largely focused on her romantic relationship with Xander, and like many characters on the show, she is portrayed as morally ambivalent.
Although Dawn's genesis is magical, she functions as a complete and normal teenage girl, and, after her true nature has been revealed, she is accepted and loved as a sister, daughter, and friend.