One of the more interesting connections made between two characters that don't seem at all related on the surface in Buffy the Vampire Slayer is the theory that Halfrek and Cecily are the same person. While this has never been addressed directly as canon, actress Kali Rocha has said on numerous occasions that she believes they are one and the same, and there is ample proof to cement this within the series.
Joss Whedon's Buffyverse is not only full of little connections and clues, but contains quite a few characters with immortal lifespans. Both Buffy the Vampire Slayer and its spin-off series, Angel, have worked to explore the often murky history of its older characters such as Angel, Drusilla, Darla, Spike, and other various demons and vampires who have become part of the greater universe. However, Spike (James Marsters) had his history revealed more slowly throughout the series, first through his relationship with his sire, Drusilla (Juliet Landau), then through his connection to Angel (David Boreanaz) and his sire, Darla (Julie Benz); finally, in season 7, he got an origin story episode of sorts, which revealed his problematic history with his mother.
Spike, who was once known as William the Bloody, ended up becoming a vampire because he was rejected by Cecily, which was addressed in season 5, episode 7, "Fool For Love". While both Cecily Addams and Halfrek/Hallie are played by the same actress, there's more to confirm the connection than just a case of double casting.
Halfrek first appeared as Cecily Addams on the show, not as the vengeance demon who is a long-time friend of Anya's (Emma Caulfield). While most fans know Kali Rocha for playing Hallie, who has a different take on vengeance than Anya - she goes after those who abuse and neglect children rather than turning her powers on scorned lovers - it's interesting that she played the role of Spike's first love before the character whom she portrayed more often on Buffy the Vampire Slayer. Though a minor role overall, Halfrek was a sounding board for Anya and a good friend before she was killed by D'Hoffryn in season 7. Since they were friends, Halfrek came to Sunnydale for Anya's wedding to Xander; this seems like a small detail, but during her stay, she recognized Spike and even called him by his former name, William. The two shared a moment of recognition, but it was never addressed further.
Realistically speaking, Halfrek is much older than Spike, which means she was no mere mortal when they first knew one another. If she was adopting the alias of Cecily Addams for whatever reason - likely to get close to a target - it would make sense that she wouldn't want to be distracted by a suitor. Her cruelty to William, which ended up setting off the chain of events that would go on to shape his entire personality as a vampire, was seemingly just a means to an end. It would be unrealistic for her to love him back; not only was she a vengeance demon, but she had business of her own to deal with, and wasn't the woman he thought she was anyway.
The greater ramifications of this action, where Cecily told William he was "beneath her", led him to consistently obsess over and berate himself for intense love and devotion to women who were, in some way, unattainable. Drusilla was mad, and Buffy never had the capacity to truly love him back the way he wanted. In season 7, episode 17, "Lies My Parents Told Me", it's shown that Spike had a similarly difficult relationship with his mother as well. She was his greatest supporter, and his decision to turn her into a vampire to save her from illness so they could be together forever had devastating results for him; the change turned her into something dark, evil, and unable to love him back. As "Lies My Parents Told Me" and "Fool For Love" are connected through a consistent narrative within Buffy the Vampire Slayer, it stands to reason that Cecily and Halfrek truly are one and the same.
