The vengeance demon who joins the Scoobies is Anya. Originally known as Anyanka, she was a former human who became a vengeance demon specializing in granting wishes to women who had been hurt or betrayed by men.
Anya first appears in Sunnydale as a seemingly awkward teenage student, but her centuries-long history gradually emerges. After Giles destroys her magical power center, she becomes human again and begins a relationship with Xander Harris. Although she initially struggles with ordinary human customs and emotions, she becomes increasingly involved with Buffy, Willow, Xander and Giles.
Anya temporarily regains her demon identity after Xander leaves her at the altar, but eventually rejects that role and returns to the group. Her practical knowledge of demons often helps the Scoobies, even though her bluntness and self-interest frequently cause friction. She dies defending the others during the series finale, “Chosen.”
D’Hoffryn is the leader who recruits and empowers vengeance demons, while Halfrek is another vengeance demon and Anya’s longtime associate. Neither joins the Scoobies as a regular member.