William "Spike" Pratt, played by James Marsters, is a fictional character created by Joss Whedon and David Greenwalt for the television series Buffy the Vampire Slayer and Angel. Spike is a vampire and played various roles on the shows, including villain, anti-hero, trickster and romantic interest. For Marsters, the role as Spike began a career in science fiction television, becoming "the obvious go-to guy for US cult [television]." For creator Whedon, Spike is the "most fully developed" of his characters. The character was intended to be a brief villain, with Whedon originally adamant to not have another major "romantic vampire" character like Angel. Marsters says "Spike was supposed to be dirty and evil, punk rock, and then dead." However, the character ended up staying through the second season, and then returning in the fourth to replace Cordelia as "the character who told Buffy she was stupid and about to die."

Within the series' narrative, William was an unsuccessful aspiring poet in the Victorian era who was mocked and called "William the Bloody" because of his "bloody awful" poetry. Sired by the vampire Drusilla (Juliet Landau), William became an unusually passionate and romantic vampire, being very violent and ready to battle, but not as cruel as his companions. Alongside Drusilla, Darla (Julie Benz) and Angelus (David Boreanaz), Giles thinks William acquired the nickname Spike for his preferred method of torturing people with railroad spikes, but it is revealed it is because his poetry was "so bad you could stick a railroad spike through your head." He was noted for killing two vampire Slayers; one in China in 1900 during the Boxer Rebellion, the other was Nicki Wood in 1977 New York, where Spike acquired his trademark leather duster. During the second season of the series, Spike comes to Sunnydale hoping to kill a third Slayer, Buffy Summers (Sarah Michelle Gellar), with whom he later forges an uneasy alliance. Over the course of Buffy, Spike falls in love with the Slayer, reacquires his soul to prove himself to Buffy and dies a hero in the show's series finale. He is subsequently resurrected in the first episode of the fifth season of the spin-off series Angel.

Considered a "breakout character", Spike proved immensely popular with fans of Buffy. The character appears substantially in Expanded Universe materials such as comic books and tie-in novels. Following the cancellation of Angel in 2004, Whedon considered creating a Spike film spin-off. Canonically, the character appears in issues of the comic books Buffy the Vampire Slayer Season Eight (2007–11), Angel: After the Fall (2007–09), Buffy the Vampire Slayer Season Nine, Angel & Faith (both 2011–2013) and several Spike limited series, spinning off from both Buffy and Angel. Currently the character is in the canonical comic Buffy the Vampire Slayer Season Eleven (2016–2017) and in Buffy the Vampire Slayer Season Twelve (2018).

Spike's story before he appears in Sunnydale unfolds in flashbacks scattered, out of sequence, among numerous episodes of both Buffy the Vampire Slayer and Angel. The first flashback occurs in Buffy season 5's "Fool for Love", and reveals William as in fact a meek, effete young man of aristocratic background (and an aspiring poet) who lived in London with his mother, Anne. Anne would often sing the folksong "Early One Morning" to her son, right up until the time he was turned into a vampire. William's surname is given as "Pratt" in the non-canon comic Old Times and is written on the label of his jar of blood in the comic Spike: Asylum #002. This surname became official with the publication of the canon comic Buffy the Vampire Slayer Season 11 #7, in which Buffy calls him by it. The name William Pratt may allude to horror actor Boris Karloff, whose birth name was William Henry Pratt, and can also be understood as the British slang term "prat", describing a person of arrogant stupidity.

Spike is one of the youngest recurring vampires on the show, though the evidence of his age is contradictory, as the concept of the character evolved over time. When he was introduced in "School Hard" (season 2), Giles read that he was "barely 200," implying that he was either born or sired in 1797 or slightly earlier. In "The Initiative" (season 4) he said he was 126, thus born or sired in 1874. Flashbacks in "Fool for Love" (season 5) show that Spike was sired as an adult in 1880. Assuming he was in his early to mid-20s when he was sired, Spike would be in his 140s during the series. In the comic Spike: Asylum #002, Spike's jar of blood has a label giving his assumed human birth date in 1853, about 27 years before he was sired.

In 1880, William was a struggling poet, often mocked by his peers who called him "William the Bloody" behind his back because his poetry was so "bloody awful." The true origins of this nickname were not revealed until three years after it was first mentioned in season 2, when it was believed to have purely violent connotations. William showed a strong capacity for loyalty and devoted love, which remained after his siring. After his romantic overtures were rejected by the aristocratic Cecily, William wandered the streets despondently and bumped into Drusilla. She consoled him, drained him of blood and made him drink of her blood, thus transforming him into a vampire – "siring" him, in the jargon of the series. Spike's grand-sire, Angelus, became his mentor (leading Spike occasionally to describe him loosely as his sire): "Drusilla sired me, but you, you made me a monster." Whereas new vampires in the Buffyverse often delight in killing their families once they become evil, William was a notable exception. Having always been very close to his mother, he turned her into a vampire to save her from tuberculosis. But his mother, as a vampire, taunted William and insinuated he had always had a sexual fascination with her. William chose to stake her because he found he could not bear to see his mother behaving like the soulless vampire he had made of her. She, like most vampires, lacked his unusual capacity for some of the softer human emotions, like love and compassion.

After staking his mother, William began a new life with Drusilla, to whom he was utterly devoted. Euphoric with his newfound vampiric abilities, he adopted the poses and trappings of a cultural rebel, affecting a working class East London accent and embracing impulsiveness and extreme violence. He adopted the nom de guerre "Spike" based on his habit of torturing people with railroad spikes – possibly prompted by criticism of his poetry: "I'd rather have a railroad spike through my head than listen to that awful stuff." In "The Weight of the World", Spike mentions having spent "the better part of a century" in delinquency, suggesting criminal activities other than killing for blood. In the company of Drusilla, Angelus, and Darla, Spike terrorized Europe and Asia for almost two decades. He had a strained relationship with Drusilla's sire Angelus, who continued a sexual relationship with her despite Spike's strong disapproval. Although Angelus did enjoy the company of another male vampire in their travels, he found Spike's recklessness and lust for battle to be unnecessary risks. Angelus regarded killing as an art, not a sport, and killed for the sheer act of evil; Spike did it for amusement and the rush.

In 1894, Spike and Angelus developed a rivalry with the enigmatic Immortal, who later had Spike sent to prison for tax evasion. In 1900, Spike killed Xin Rong, a Chinese Slayer while in China during the Boxer Rebellion. In 1943, he was captured by Nazis for experimentation and taken aboard a submarine, where he was briefly reunited with Angel. By the 1950s, Spike had reunited with Drusilla, and they traveled to Italy. At some point, Spike also became a rival of the famous vampire Dracula. The enmity between Spike and Dracula was explored in the comic series Spike vs. Dracula, in which their mutual hatred is caused when Spike, along with Darla and Drusilla, slaughtered the Romani (gypsy) tribe who had cursed their patriarch Angelus with a soul, although it is unclear if either Spike or Drusilla knew precisely why Darla was so angry with the tribe. That clan (unknown to Spike) was favored by Dracula, and he sought revenge for their deaths. Spike later mentions in a conversation with Riley Finn, "Dracula? Poncy bugger owes me £11, for one thing," because Dracula tossed Spike's signed copy of Bram Stoker's Dracula in a fire in 1898. Spike also notes that Dracula's fame has done more damage to vampires than any Slayer, since it made their weaknesses more widely known. Spike attended Woodstock in 1969, whereupon he accidentally became high after ingesting the blood of a hippie. He claims to have spent the six hours following the incident "watching my hand move". In 1977, he fought and killed a second Slayer, Nikki Wood, aboard a subway train in New York City, taking from her a black leather duster which he wears throughout his appearances on Buffy and Angel until it is destroyed in an explosion in season 5 of Angel, whereupon he gets a new one that looks exactly like the old one ("The Girl in Question"). At some point post-1977, Billy Idol allegedly "stole" Spike's look and made it famous as his own (as revealed in season 7’s "Sleeper"; see "Appearance" below); Spike's thoughts on this are unrecorded. Inasmuch as Buffy knew of the "theft" as of "Sleeper," Spike presumably shared the detail with her in an undepicted moment.