David Boreanaz plays Angel, the 242 year old vampire whose in love with *who else?* the slayer. How is this possible you say? Well, when Angel killed a gypsy princess a curse was put on him to give him a soul. Since regular vamps don't have a soul, they kill people and don't care. But not Angel. Since his change he hasn't had a single human.
Wait!-That was before Buffy and Angel took their forbidden relationship to another level. Angel received on moment of happiness and his soul was taken away again *sob*. But! At the end of Becoming, Willow again restored his soul, but it was too late. Buffy could only close the gateway to hell that Angelus had opened with his blood! Don't worry though, she staked him with a sword, and we all know that a vamp can't by killed by steel.
In season three, Angel was released from Hell as a ferocious beast. Thanks to some help and guidance from Buffy, Angel returned to his old soul self again. Angel continued to help out Buffy and the Scooby Gang throughout season 3, but he has just recently decided to leave Sunnydale at an attempt to give Buffy a normal life.