Spike

1880, William was a young English gentleman. In love with a woman named Cecily, he wrote poetry entirely devoted to her. When finally has the courage to declare his love to her, she is appalled and violently rejects him. He flees the scene and disappears, crying, into the dark London streets. There he bumps into Angelus, Darla and Drusilla. Drusilla follows him into a dark alley and makes him a vampire. For over one hundred years, they travel the world together and Spike has eyes for her only. In Prague, in the late 1990's, they are attacked by and angry mob, and Dru is severely weakened. Spike brings her to Sunnydale hoping to heal her.
There he meats Buffy, and reveals his obsession for Slayers. For years he tries to kill her and ends up being captured by the Initiative that place a chip in his head that prevents him from hurting humans. Needing his share of violence, he reluctantly joins the Scoobies fight. As he falls for Buffy, he ends up getting a soul. Manipulated by the First he starts killing again, but Buffy manages to stop him and he plays a huge part in the fight against the supreme evil by giving up his (un)live during the battle and saving the world.
Although he was seen burning up in Chosen, Spike will return to Angel next season.