Spike
When we first saw Spike step out of his car, we all knew this character was going to provide a lot of humour and entertainment.
William the bloody, nicknamed Spike for his use of railroad spikes as murder weapons, was sired by Angel just under 200 years ago.
Although Spike is a vampire, he has no respect what so ever for tradition and authority (displayed when he murdered the "annoying one"). He does what he likes, when he likes.
"Don't get me wrong, I love a good slaughter as much as the next guy." Passion
The only person, (undead of course) that he will listen to is his mad girlfriend Drusilla (Dru).
Spike and Dru came to Sunnydale to help restore Dru's health. What they didn't know, is that Angelus, their old friend turned nemesis was already living in this town, the Boca del Inferno.
Spike's first attempt to kill Buffy on the night of parent teacher interviews ended with him being hit over the head with an axe. Thus ending his record of murdering each Slayer he has fought (that's two slayers). The rest of his attempts to destroy Buffy had the same success less results.
And his luck didn't get much better. While trying to restore Dru's health, Spike was injured and restricted to a wheel chair, relying on Dru's help to survive.
But it wasn't until he was trying to destroy the world through a demon named the Judge, that he was reunited with as old friend. The soulless Angelus. Although the Judge failed, Spike was pleased to have Angelus fighting on their side once again. But this all changed when Angel took advantage of Spike's disability, and took over his position as Drusilla's "playmate".
Spike finally deciding he had, had enough, teamed up with Buffy to destroy Angelus and to gain Dru again.
While Buffy and Angel were battling it out, Spike took Drusilla away from all of these troubles, vowing he would never return. Several months later, we saw him stumble, literally, back into town, drunk and planning revenge against all the people/demons who had anything to do with Dru breaking up with him.
He kidnaps Willow and Xander, ordering Willow to perform a love spell which will make Dru's feelings towards him return.
Eventually he sees what sorry state he is in and comes to the decision that winning back her affection personally, is the only way to go. So, once again we see him leave, but this time we know it's not for good.