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).
Spike was born as William Pratt between 1850 and 1853 to Anne Pratt and an unidentified husband. By 1880, William was a sensitive and ineffectual gentleman still living with his mother. He made his living as a poet, and he and his mother lived comfortably. William was desperate to make a name for himself in London's high society. His poetry was ill received however, and earned him the nickname "William the Bloody", a name that would follow him into his future as a vampire (albeit with far deadlier conotations). William was regarded as a fool by his contemporaries. Eventually, he confessed his love for the aristocrat Cecilly Addams who painfully rejected him. Distraught, William roamed the streets in misery, bumping into Angelus, Darla and Drusilla, a trio of deadly vampires.Drusilla had been aching after a "playmate" to call her own, tired of sharing Angelus with Darla. She decided to sire William, and followed him into an alleyway, seducing him and turning him into a vampire. The pair spent the next couple of days in a frenzy, drinking their fill of blood and alcohol. William eventually wished to return to his mothers house, hoping to save his mother from dying of tuberculosis by turning her into a vampire. He did so, but found that as a soulless vampire, his mother was a very different person. She taunted him, claiming that she had despised him all along and that he was sexually attracted to her. Horrified and unwilling to see his mother in such a twisted light, he killed her.
William gave himself over wholly to his new life with Drusilla, who introduced him to her sire, Angelus. The still innocent William initially ideolized Angelus, who in turn was interested in the prospect of living and hunting with another man, having spent his 100 years with women alone. William would soon be taught a harsh lesson by Angelus however, when he walked in on Angelus having sex with Drusilla. William realized that Angelus had done this to hurt and desensitize William upon learning the extent of William's infatuation with her. William angrily declared Drusilla to be his "destiny," only to be mocked by Angelus, who declared that concepts such as "deserving" and "belonging" did not matter in the world of the vampires. As a vampire, William was free to take whatever he desired, but could never truly have anything. William would later remember this as the beginning of their rivalry.
As part of his efforts to keep Drusilla's attention and affection, William gave up all remnants of aristocracy that remained in him, adopting a working class way of speaking, and giving himself over to impulsiveness and violence. He took on the nome de guerre of "Spike," which is how he would become known for the rest of his life. Together with Drusilla, Angelus and Darla, Spike spent the next 20 years terrorizing Europe and Asia, remaining utterly obsessed with Drusilla, yet growing to hate and envy Angelus, who in turn began to grow sick of Spike's eagerness for pointless battles. To Angelus, murdering was an art to be perfected. He delighted in destroying his victim's minds as well as their bodies, and avoided fights that posed a risk to his wellbeing. Spike on the other hand simply killed people for the rushed, and believed that the more dangerous a fight, the more exhilirating. Despite their growing rivalry, the two made an extremely formidable team, battling many opponents, among them the Immortal.
By the end of the 19th century, the Whirlwind as the four vampires were called would begin to come to an end. During a massacre in Romania, gypsies restored Angelus' soul after he killed their most beloved daughter. Spike unknowingly ruined any chance of reversing the curse by killing the hostages Darla were using to blackmail the gypsies with. Disgusted by Angelus' re-ensoulment, Darla rejected and exiled him. Spike was not unhappy, and attempted to convince Drusilla to forget about Angelus.
Angelus would return however, rejoining them in the Boxer Rebellion two years later. There, Spike fought and killed his first Slayer, having developed an obsession with them ever since being told the story by Angelus. Angelus congratulated Spike on his acheivement, but ultimately proved unable to participate in the massacre, his soul weighing too heavily on his conscience. He left once more, and Spike and Drusilla lost contact with Darla.
The pair continued their lives of murder and mayhem, with Spike killing his second Slayer in 1977, stealing her leather jacket which would become a mainstay of his own wardrobe. In 1997, the pair were attacked by an angry mob, and Drusilla was grievously wounded.
Spike and Drusilla travelled to Sunnydale, in the hopes that the Hellmouth's energies would help Drusilla recover. What made the location even more enticing was the presence of a Slayer, whom Spike hoped to make his third kill. To his dismay, his first attempt was thwarted by none other than Angelus, now going by the name of Angel, who had allied with the Slayer, Buffy, in hopes of repenting his murderous past.
Spike eventually discovered a ritual that would cure Drusilla, requiring the blood of her sire; Angelus. Spike was more than happy to kill his former mentor, and captured him, beginning the ritual. Buffy manged to interrupt it and save Angel, crushing Spike under a pipe organ, crippling him in the process. However, the ritual still succeeded, and Drusilla was restored to full health.
In planning for Drusilla's birthday party, Spike ordered his minions to gather the pieces of the Judge, an ancient and all-powerful demon capable of killing any non-demonic life with a single touch. Spike planned to resurrect the demon for her birthday. In the process of trying to stop Spike, Angel and Buffy grew closer than ever, and took their budding relationship to the next level by sleeping with each other. The moment of perfect happiness that Angel experienced during sex broke the gypsy curse, making him soulless once again.
Angelus sought out and joined Spike and Drusilla, making plans for destroying humanity with them. Spike was initially overjoyed at his old mentor and grandsire's return, but their relationship soon soured once again over arguments on how to destroy Buffy. Angelus repeatedly gave up chances to simply kill her, wishing to completely and utterly destroy her mind and body first, while Spike advocated simply ending the threat she posed. What drove Spike over the edge however was the fact that Angelus resumed his sexual relationship with Drusilla with Spike incapacitated. Spike soon recovered, but remained in the wheelchair, feigning weakness in order to plot against Angelus. After Angelus unveiled his plan to summon the demon Acathla to suck Earth into a hell-dimension, Spike made his move, seeking out Buffy and offering an alliance. Spike claimed that he wanted Drusilla back, and that he didn't want to see the world end, since he enjoyed living in it so much.