Simone Doffler is a rogue Slayer and an antagonist in the canonical comic series Buffy Season Eight - she is also the "BIg Bad" of Season Nine.
History
Simone Doffler was a member of Rona's training squad based in Chicago. Despite her strength and talent in battle, she wasn't particularly well-liked among her peers, one of whom pointed out that she would not be willing to risk her life for others. Eventually, Rona transferred Simone to Andrew Wells' squad in Rome, believing that a less urban setting might soften her "rough edges". Buffy would later describe this as Rona "passing the buck" onto Andrew.
Simone continually expressed a desire to use firearms instead of traditional Slayer weapons. Andrew dismissed her for even suggesting this, insisting that Slayers did not use guns. Later on, Simone met three like-minded Slayers and finally went rogue. Alongside them, she broke into a weapons storage facility and stole caché of firearms, including machine guns and ammo. The Slayer Organization attempted to track, but she had gone off the grid.
Buffy and Andrew joined forces to track down Simone who had amassed a full-sized squad of her own, with whom Simone had taken over an entire island and expelled its original inhabitants from the island's town. She rescued her subordinate, Nisha from a trap set by Andrew and escaped with a massive Ragna Demon bred by Andrew, intending to use it as a weapon.
Like Simone expected, Buffy went to the island to confront her. Once the two met inside Simone's seat of power, the town's opera house, she offered Buffy the Ragna Demon, which she nicknamed "Arachnophobia", in exchange for Andrew, whom Simone had grown to hate while he had been her Watcher. Buffy, however, refused to leave without Andrew even if she recuperated the demon.
Simone refused to let Andrew go and reminded Buffy that she was outnumbered sixteen to one. However, at that moment, the Italy Squad showed up to rescue their Watcher. The Squad refused to fight against Simone and her followers in fear they might employ their firearms against them and chose to leave to fight another day. Uneasy with leaving the island to Simone, Buffy released the Ragna Demon so she would do some damage to Simone's operation.
Simone's criminal activities, combined with the machinations of the Twilight Group, the new Harmony Bites reality show hosted by the vampire Harmony Kendall, and the destruction of Sunnydale, have resulted in a new pro-vampire, anti-Slayer world order; now viewed as terrorists worldwide, the Scooby Gang and the Slayer Organization have been forced into hiding.
Simone would return in Season 9 following the dramatic destruction of the "Seed" during the Twilight crisis, which destroyed most magic in the world - due to a lack of vampires, demons or witches (save for "zompires") Simone would become the new "Big Bad" of this season.