A cocky vamp from 19th century England, Spike got his former (pre-bloodsucker) name, William the Bloody, because of his bloody awful poetry. However, in 1880, the sensitive lyricist was sired by his immortal lover Drusilla and got the name "Spike" because of his love for torturing victims with railroad spikes. Not exactly a Victorian poet a la William Wordsworth, Spike has killed two Slayers. The first of these was a Slayer during the Boxer Rebellion and the second was a punker kid in New York-whose trench coat has become synonymous with Spikes predatory attitude.
One of the most interesting members of Sunnydale's night stalkers, Spike found himself off and on with Drusilla who finally ended up leaving our favorite baddie the first time for the Chaos Demon and the second time for the Fungus Demon. According to Dru, the Victorian poet turned psychotic Slayer killer just wasn't evil enough for her. A traveler, the killer returned to Sunnydale one year trying to find the Gem of Amarra that would make him invincible. Eventually, Spike began dating former Sunnydale High snob turned fang face, Harmony. That didn't work out and Spike has taken a vested interest in our heroine Buffy. To his chagrin however, the Initiative implanted a chip in Spike's head to make him harmless to humans. More recently, Buffy told Spike that she found him disgusting, saying, "The only chance you had with me was when I was unconscious."
During the final battle with Glory that lead to the Buffster's death, the inherently vicious Spike kept his word to protect Dawn. Don't expect Spike to take on all of the values of the Scooby Gang·but expect his love for the fallen Slayer to drive his bloodlust in a more acceptable direction.