William the Bloody or "Spike" (as dubbed for his propensity for torturing victims with railroad spikes) was originally a pure villain introduced on Buffrialth paramour, Drusilla. Originally a timid Victorian poet, William was turned by Dru in 1880. As a vampire, he wreaked havoc and mayhem with Angel's clan throughout Europe. He also has the notorious distinction of killing two Slayers - one during the Boxer Rebellion in China and the other in 1977 in New York City.
Spike and Dru came to Sunnydale by way of Prague in search of a way to restore Dru's frail health. After a brief reunion with Angelus and many unsuccessful run-ins with Buffy, the couple fled to South America where they eventually ended their relationship.
Spike returned to Sunnydale where he was essentially rendered harmless at the hands of the secret, government demon hunting operation known as The Initiative. Their scientists implanted a chip in his head, which made it impossible for him to attack humans. In a slow evolution over three seasons, Spike became a quasi-Scooby member reluctantly helping the good guys because of his love for Buffy.
Yes, the self-proclaimed "Love's Bitch," who has always been led by his heart fell in love with the Slayer. Initially refused by Buffy, they eventually engaged in a dark, sexual relationship, which almost destroyed them both. Distraught at their break up, Spike attempted to rape Buffy and though unsuccessful, he was devastated by his actions. Spike then sought out to become what he once was and through a series of hellish trials in Africa, he earned a soul.
Spike returned to Sunnydale with an extreme bout of insanity due to the guilt of his demonic history. Slowly, he regained his sanity and earned Buffy's forgiveness as well as a place at her side fighting The First Evil. In an action of ultimate self-sacrifice, Spike finally attained redemption as well as his purpose by sacrificing his life to close the Sunnydale Hellmouth from the armies of The First. Wearing an amulet provided by Angel, Spike burned to ash at the bottom of a crater - now the last remains of Sunnydale.