One of the often-discussed mysteries of Joss Whedon's cult classic TV show, Buffy the Vampire Slayer, is how the Slayer line works. For example, a point of contention for some fans is why, exactly, a third Slayer wasn't called after Buffy died in the season 5 finale, "The Gift". Though it might seem like a plot hole in some regards, that's actually not the case.
Buffy Summers (Sarah Michelle Gellar) was known for saving the world... a lot. The Chosen One, gifted with once in every generation powers from a rich legacy that dates all the way back to the prehistoric era, Buffy's calling is to fight vampires, demons, and the forces of darkness. While the concept of the Slayer is meant to seem like a long-held mantle, the lifespan of a Slayer is notoriously short. Given they must put themselves in harm's way and often die saving the world, the torch is continually ed from young woman to young woman, with the next in line activating upon her predecessor's death. In season 1, Buffy dies for the first time at the hands of The Master, fulfilling a prophecy that she tried, initially, to avoid.
Though Buffy is quickly resuscitated, even the temporary death was enough to call the next Slayer in line, Kendra Young (Bianca Lawson). Kendra was from Jamaica, and had a very different outlook to being a Slayer than Buffy. Although Buffy took her duties seriously, and was dedicated to her training, she was equally focused on living a relatively normal life, complete with school activities, friends, and even a vampire boyfriend, Angel (David Boreanaz). Kendra was militant in her training, absolutely dedicated to the cause in a way that placed nearly everything else on the back burner; even so, she met a quick death at the hands of the vampire Drusilla (Juliet Landau) in season 2. After Kendra's death, the Slayer line didn't just revert back to Buffy—a new Slayer was called. Due to this progression, Buffy's second death in season 5 makes more sense than one might think.
Why A Third Slayer Wasn't Called After Buffy Died In "The Gift"
Faith Lehane (Eliza Dushku) made her first appearance in season 3, episode 3, "Faith, Hope, and Trick". Eventually known as the "dark" or "rogue" Slayer, she was incredibly different than both Buffy and Kendra, showcasing just how diverse the Slayer line is. Prior to season 7, it was largely unknown what constituted someone as worthy of being called as a Slayer. The Potential Slayers, or young women who were waiting in line to be called upon the current Slayer's death, all came from different walks of life, had very different personalities, and some of them almost seemed like they wouldn't be strong enough to carry out the duties, should they be next. Faith's rough and tumble attitude, enjoyment for fighting and killing, and reckless lifestyle choices made her an obvious foil for Buffy; the two had a complicated and violent rivalry, but eventually worked toward patching things up in season 7.
However, Buffy's death in season 5 was far more permanent than her initial death in season 1. For that reason, many wondered why there wouldn't be a third Slayer called. Although Faith was still perfectly capable of upholding her duties, the show's story was largely focused on Buffy as the Slayer. In many ways, it still seemed like she was carrying the torch alone, like she was still the Chosen One, even though she technically wasn't. The answer to why there wasn't a third Slayer called is actually quite simple: the Slayer line continued through Faith, not Buffy. While Buffy came back to life in season 1, for the sake of the line's continuity, the torch was ed irrevocably; she retained her powers upon resurrection both times, but the gift was never solely hers again.
One possible suggestion that has been made about why Buffy behaves like she's the one and only is because, not only do other people treat her as if she is, but she has gotten so wrapped up in her calling and mission that she can't see it any other way. She's the star of the show, the focal point of Buffy the Vampire Slayer, and while it acknowledges that other Slayers exist, it's ultimately her story. Season 7 changes this up by completely opening the Slayer line to include all Potentials, but even before that, Faith was the true successor. Upon her death, a new Slayer would have been called.