 Buffy 's former nemesis is Spike ( James Marsters ) , a vampire . In the fourth season The Initiative , a secret military organization whose mission is to evaluate and eliminate demonic beings , rendered Spike harmless by implanting a microchip in his head that causes him intense pain when he attacks humans . However , the chip does not affect him when he harms demons and he now often fights on Buffy 's side , after at first fighting just for the pleasure of brawling . His motivations changed when , in the fifth season , Spike realized he had fallen in love with Buffy . She initially rejected him , but after and just before her death they had begun to form a friendship of sorts . She has been confiding in him ; prior to this episode , he is the only one to whom Buffy has revealed that she was in heaven . 
