Spike and Dru arrives in Sunnydale. The first thing he does is to kill the Annointed One. After that he wants to kill the new slayer. But he underestimates Buffy - and her friends. What he doesn't know: she has help from his sire - Angel. He stays in Sunnydale longer than he had planned. With some 'help' from Angel he can heal Drusilla, but during the fight, when Buffy wants to save her boyfriend, Spike gets buried under a big organ. Now Drusilla is fine, but Spike has to sit in a wheelchair. So Dru cares for him, but only until Angel loses his soul and comes back to his old friends. Angelus and Dru have plans to destroy the world, Spike just watches them and doesn't like their plans at all. Of course, all plans fail because of Buffy. But at the end, they have a perfectly save way - 'Acathla'. Spike likes the world and he likes Drusilla even more, so he has to work together with Buffy to fight against Angelus. So after the big fight, Spike leaves with Drusilla and goe to Brazil and Buffy has to kill Angel who - in the meantime - got his soul back.
He promised not to come back, but he lied. After Dru left him for a chaos demon, Spike comes back to Sunnydale, totally drunk. He forces Willow to do a love spell so that Dru would love him again. Of course, Spike stumbles about Buffy and Angel and he almost drives them mad with what he says - but it's the truth about their realtionship. After a little fight with old fellows he leaves without the spell, singing "My Way" and leaving two quite confused people.
And he's back again. Actually he just came back to Sunnydale to search for the 'Ring of Amarra', Buffy wasn't supossed to know that he was there. But they both meet on a party and Spike's ring is as good as lost. Just when he wants to take revenge on Buffy, the Initiative captures him. Spike comes free, but now he has this bloody chip in his had which brings him horrible pain each time he wants to hurt a human being. And so it happens that Spike depends on the help of Buffy and her friends. First he 'lives' in Giles' bathtub and later in Xander's home, before he gets his own crypt. His former girlfriend, the blond and naive Harmony has left him in the meantime to start up her own vampire gang. At this time Spike helps the Scoobies more or less...
After a whole while Spike starts being nice to Buffy. He helps if he can and takes care of Dawn and her mother. When Buffy gets to know that he fell in love with her she doesn't want to see him again. No matter what he does to prove his love, Buffy doesn't care. When Glory kidnapps and tortures him to make him tell her who the 'key' is, he doesn't say a word to protect Dawn.When Buffy hears about that and when she sees him, she's very grateful and kisses him. Spike hopes that he at least has a little chance now, but all his hope is gone when Buffy dies in the last fight against Glory After her death Spike cares for her little sister Dawn.
Buffy is brought back to Willow by Willow and a pretty dangerous spell. Spike is the only one who can understand her feelings. As time goes by Buffy loses his anger against Spike and visits him quite often in his crypt. After Buffy and Spike expressed their deepest feelings caused by a spell they kiss for the first time. They go as far as they could and after a really hot night Buffy is as cold as she's been before. It's gets even worse when she breaks up with him, cause she thinks that this realtionship is not what she's been searching for. When Spike tries to talk to her Buffy rejects him again and again and even insults him. Spike freaks out and tries to rape her. She can stop him just in time and when he realizes what he was up to do he runs of in panic and leaves Sunnydale. His plan was a trip to Africa to get rid off his chip and then Buffy. But there seemed to be a misunderstanding, when Spike says he wants to be like he was before. He didn't mean to become William again - and he certainly didn't mean to get his soul back...
Spike came back to Sunnydale after he was ensouled. Where else should he go... It took Buffy some weeks (?) to find him by accident. He was living in the basement of the new built highschool. When rescuing some students Buffy stumbled across him. Of course she didn't know about his new soul, and he was too confused to tell her. He wasn't able to talk straight, so she left again, thinking that he was now completely insane. It took him some more weeks to handle his new soul, but in the meantime he didn't miss to tell Buffy about it. She was more than shocked, rather moved to tears when she realized why he did it.
Of course, it took Buffy some time to trust him again, but finally she did. So they started to work together again. Soon Buffy had to proof how much she trusts him when Spike was posessed by the First Evil, this season's Big Bad. But no matter what he did and what happened, but always stated that she needs Spike, that he's her only real help when fighting The First Evil. Even when he tells her that he wants to leave Sunnydale to avoid being a great danger for her and the Scoobies she says that she's not ready for him to leave her *so sweet ;)* When Spike's chip starts to act weird Buffy takes him to the former Initiative halls where they find some remaining soldiers. Buffy has to decide whether she wants the soldiers to repair or to remove Spike's chip. She decides for the second possibility. When Giles finds out he kinda freaks, and now he and Director Wood think that he's an even greater danger. When their plan to kill Spike fails, Buffy makes clear that she would choose Spike, not Wood, Giles or anyone else...
So it is understandable that she gives Spike the medallion that only a champion may wear. And indeed Spike proves to be worthy and saves the world and sacrificing himself.