Spike is a powerful vampire and a recurring villain turned anti-hero in the television series Buffy the Vampire Slayer as well as its spinoff series Angel - he would also play an important part in the extended universe after the television series ended in the form of comics (Spike is popular enough to even earn his own limited series).
Although Spike has been on the side of the protagonists for some time now, he has always been a wild card and was never fully trusted (for good reasons). Unlike Angel, Spike never really repented for his misdeeds and did not turn good of his own free will - though in time Spike would start to become a more heroic character.
Prior to this Spike was a merciless murderer who came to Sunnydale alongside the criminally insane Drusilla in order to kill Buffy. Spike is a rare example of a vampire who has reportedly killed several Slayers and although he failed to kill Buffy, the two developed an unusual relationship later in the series that was fairly infamous for being abusive (in some ways a dark mirror of her former relationship with Angel).
He is portrayed by James Marster who also portrayed Brainiac in Smallville, Lord Piccolo in Dragon Ball: Evolution and Zamasu in the English dub of Dragon Ball Super.