In 'Angel', season 1, after kissing Buffy for the first time, he shows his vampiric face, and scares the life out of Buffy. He helps her along the way, but in a minute of passion, he loses his soul. He tries from there on to torture Buffy and her friends. He killed Jenny Calendar in the episode'Passion'. In 'I Only have eyes for you', both him and Buffy are inhabited by ghosts and kiss,but he pushes her away.
Hope floats though.
Buffy is just about to kill him when a spell cast by Willow restores his soul. They kiss, and declare their love for each other,but then, she kills him so that the world can be saved. He returns in one of the early episodes of season 3, after hellish torture. Buffy breaks up with Scott, and turns to Angel instead. But later, he leaves.