Warning: Wikipedia contains spoilers.
Darla, a vampire, is the longtime lover of Angel. However, while he was given a soul, she remained a typical vampire - an evil, soul-less killer. Nevertheless, in Season Three of Angel, certain events led her to become pregnant with Angel's child (later called Connor), and she recognised him as the one good thing she had ever created. In her final act, she kills herself in order to give birth, dissolving into dust around the new-born child.