Cassie Newton, who first appears in the seventh season episode "Help", is a young girl who comes to Buffy and tells her that she (Cassie) will die on the next Friday. Buffy, along with Xander, Dawn and Willow, manages to save her from a group of boys who try to kill her in order to raise a demon. However, immediately after, Cassie dies of a heart attack caused by a family condition, fulfilling her own prophecy (this is a near perfect match to the story of Cassandra in Greek mythology.) She does, however, leave several prophecies relating to the Buffy final episode -- including telling Spike "She'll tell you" and implying to Buffy who would win the final battle with the First.
Cassie then appears to Willow in "Conversations with Dead People" and tells her that she had a message from Tara; that if she uses magic, she will kill all her friends. Willow believes her until Cassie tells her that she should kill herself to save everyone and be with Tara again. Willow then realizes that this "girl" is not Cassie. After being found out, "Cassie" (who is really The First Evil) becomes cocky and tells Willow that "she" is going for a big finish, and disappears.