In Canon Edit
Spike is a sarcastic bad boy who once set himself the mission to kill Slayers. He is a formidable antagonist to Buffy and her friends in his first appearances. However, his encounters with Buffy leave an impression on him, and he becomes obsessed with her. Although he can't truly love her, being a soulless demon inhabiting a human body, his obsession leads him to fight with her against other foes, and he begins to desire a real relationship with her. He even goes so far as to attempt to get his soul back to win her over.
In various flashbacks, it is revealed that in his former, human life, William was a soppy romantic who wrote "bloody awful" poetry and was unusually devoted to his ailing mother. His first act as a vampire was to turn her in an effort to save her from tuberculosis, but she became as emotionless and cruel as most vampires, and he staked her himself.
In Fanfiction Edit
Because Spike was very friendly with Angelus and is known to have been part of his "family" (sometimes known as the "fanged four") back in the bad old days of Victorian England, it is assumed by many writers that the two were (and are) lovers. Spike usually plays the role of uke in this relationship. Some Spike/Angel slash is goodfic, but a vast quantity is badslash featuring PWP, rape, and dubious lube.
Buffy/Spike is one side of a once-momentous shipping war, with the other side favoring Buffy/Angel.