Angel (season 1)
At the end of the third season of "Buffy", the 242-year-old, re-ensouled vampire Angel left Sunnydale to give teenage Slayer Buffy Summers the chance to live a normal life. Angel is now living in the big city of Los Angeles. With Buffy gone Angel is now completely cut off from society, every day getting closer and closer to giving in to his vampiric hunger. It's not until he befriends the half demon and fellow Irishman Doyle, who is sent visions of people in trouble by The Powers That Be, and fellow Sunnydale resident Cordelia Chase, who grounds Angel's life in the humanity around him, that he truly begins to take charge of his life and seek atonement for his past sins. He sets up his own detective agency, Angel Investigations and begins to "help the helpless".
Faith (Buffy the Vampire Slayer)
In the "Angel" season one episode "Sanctuary", Faith forms a bond with the vampire-with-a-soul Angel; executive producer David Greenwalt explains Angel can help her because he alone is able to understand the suffering she has been through and how to help her atone for her sins. Faith is then able to return the favor in "Angel"'s fourth season, when she is the only one determined to defeat Angel's soulless alter ego Angelus without killing him in the process. Actor David Boreanaz explains, "I think having a character like Faith come back at a pivotal point when she finds out Angelus is loose is really, for her character, a way of saying: 'I'm paying you back, Angel, for saving me, therefore, I'm gonna save you.'" The writers believed it would be an interesting dynamic to have former "bad girl" Faith play a heroic role against the show's now villainous protagonist, Angel.
Faith was brought back for the final season of "Buffy", because, according to David Solomon, "she had been such a crucial character at a very specific junction in the series that there would be no way to tie it up without her." However, in season seven the dynamic between the two Slayers has changed. As Rebecca Rand Kirshner explains, "[Faith] is no longer such a complete opposite of Buffy. And there's sort of a subtler and more complicated dynamic between them". As Buffy struggles with her unwanted position of mentor to the Potential Slayers, the writers used Faith to create an outside conflict about Buffy's leadership abilities. Although Faith is questioning Buffy and her choices she is making, she is no longer an enemy to her either. In the episode "Empty Places", the Potentials lose trust in Buffy and appoint Faith as their leader instead, a decision that literally blows up in their faces. As Drew Goddard explains, "Faith is like the cool aunt that everyone loves, because the cool aunt doesn't have the responsibility of raising the children. She just gets to show up and have fun. The problem is, Faith is not ready to lead. She's damaged in her own way. She's just beginning to pull herself together. As much as she wants to be Buffy, she has to learn how to become Faith."