Buffy the Vampire Slayer/Ethan Rayne
Ethan Rayne is a chaos magician from Buffy the Vampire Slayer who is tied to the lurid past of Rupert Giles. After the two friends had a falling out, Rayne established himself as a magical subcontractor who would accept jobs from all kinds of humans and demons without questions. Rayne was played by Robin Sachs and appeared four times in the series, plus two comic appearances.
Ethan Rayne's formative years in Britain remain largely unknown, the furthest back the series went with Rayne was the 1970s when he was a small time wizard who befriended Giles, then going by "Ripper," in the underground occult scene. The two became fast friends due to their shared appreciation for small magicks until they attempted to step beyond the high they received from amateur wizardry by allowing a demon named Eyghon inhabit their bodies for short whiles. Rayne, Ripper, Philip Henry, Diedre Page, Thomas Sutcliffe and "Randall" all had the beast's mark tattooed on them and took turns letting themselves be taken over by the demon and sending it back with an incantation. However, the rites which banished the demon didn't work on Randall and he was killed in the experience.
Rayne and Giles split their friendship at this point, Giles opted for training at the Watchers' Council and Rayne continued to pursue more and more dangerous magical avenues, before eventually settling on "worshiping chaos."
Many years later, Rayne set himself up as a go-to person for magical tasks and crossed paths with Giles and Buffy on an infrequent basis. One Halloween, Rayne set up a costume shop in Sunnydale which transformed the wearers into the identity of their costume (i.e. Xander turned into a soldier, Willow a ghost, Buffy a timid princess). Giles recognized Rayne's MO and foiled his plans, but failed to apprehend him.
Several weeks later, Rayne surfaced again to confront Giles about The Mark of Eyghon and the return of the demon which fractured their friendship so many years ago. The demon possesses the corpses of its victims and hones in on more victims through the marks which are tattooed on Giles and Rayne's bodies. In order to prevent the demon from killing him, Rayne knocked out Buffy and tattooed the mark on her while removing the symbol from himself using acid. The demon was distracted from pursuing Rayne, but it didn't have time to kill Buffy, as her friends appeared in time to stop him. Rayne escaped in the scuffle.
Rayne doesn't come back until the next year when The Initiative had revealed itself to Buffy. Rayne warned Giles of the Initiative's attempts to throw chaos and order completely out of balance and poisoned his beer with a tablet that knocked him out, allowing Rayne to turn him into a Fyarl Demon. He then tries to convince Buffy that the demon killed Giles and is now after him, but she sees through his ruse and forces him to turn Giles back to normal.
After this instance, Rayne didn't manage to sneak away, however. Instead, Riley Finn placed him under military custody for rehabilitation in a Nevada facility. Though Finn surmised that Rayne would only be under arrest for a few months, he remained under military control for over three years.
During this "rehabilitation" period, Rayne was never released into the world, probably because of the knowledge he possessed that would aid the slayer and her band of slayer trainees against a coming conflict between the supernatural and humans. When Buffy was placed in a magic-based coma by Amy Madison, Raine's metaphysical form surfaced in Buffy's sub-conscious to help her decipher some symbols in her dreamscape. She eventually wakes from the coma and, with Slayer trainee Satsu, lays siege to a military base which has kidnapped Willow.
In the base, Buffy discovers a door bearing the number 30 on it. She recalls seeing Rayne trapped in a cage of three X's in her dreams. She breaks down the door, but is too late to save Rayne. He was shot in the head by General Voll, who knows that he was the only person that could help her against "twilight."
Additionally, Ethan Rayne made two appearances in the "season eight" comics, albeit his second appearance was only of his corpse.