Spike (born William Pratt) was a famous and widely feared vampire, well-known among both humans and demons for having faced and killed two Slayers throughout his unlife and for his history of torturing his victims with railroad spikes (which is what it is rumored the moniker of "Spike" is derived from); in fact, his reputation for evil and bloodshed was second only to that of his grandsire Angelus'. In his human life, William was an unsuccessful and unappreciated romantic poet known as William the Bloody, for his "bloody awful" poetry, though after he became a vampire, that moniker was repurposed to imply he was very violent. After being sired by Drusilla, he reinvented himself and terrorized Europe alongside Drusilla, Angel and Darla throughout the 19th and 20th centuries.
Spike first traveled to Sunnydale in 1997, accompanied by Drusilla. Though he quickly established himself as one of Buffy Summers' most dangerous enemies, he was eventually forced into multiple uneasy alliances with her and her team, the Scooby Gang, after he was captured by the Initative and implanted with a cerebral microchip that rendered him unable to harm humans. Eventually, Spike realized that he had fallen in love with Buffy and officially joined the Scooby Gang, later being motivated by his love for her to successfully fight to regain his soul.
Spike sacrificed his life in battle against the First Evil, both defeating its army of Turok-Han and permanently closing the Sunnydale Hellmouth, as well as the town of Sunnydale itself. However, bound to an ancient amulet, Spike became an incorporeal entity haunting the halls of the Los Angeles branch of Wolfram & Hart, then under the management of Team Angel. After becoming corporeal again, Spike struggled to find a place in the world, only to join Angel in his battle against the Senior Partners and become a Champion in his own right.
After a slew of adventures, Spike reunited with Buffy and the Scoobies in Europe and joined them in the battle against "Twilight." When the world was made void of magic, he moved to San Francisco to continue fighting by Buffy's side, but eventually left in the hopes of "finding himself."