Sent over from England, Giles is Buffy's new Watcher. He trains her and helps her in her fight and soon becomes a sort of substitute father to her. The only adult of the gang, he tries to have authority, although no one really listens to him.
Fired from the Council for being too close to Buffy, Giles stays in Sunndyale and helps the gang. He is considered the 'brain' since he's very much up-to-date on the occult and has an extensive collection of books about it.
Although Giles seems very proper and responsible, he was quite a rebellious teen, nicknamed Ripper. Opposed to his destiny of becoming a Watcher, her messed around with the Black Arts, summoning demons and eventually killing one of his friends.
He will do whatever it takes to win the fight - even kill humans and allies. He killed Ben in 'The Gift' and was all willing to kill Spike in 'Lies My Parents Told Me'.
Feeling he has nothing more to teach Buffy and that she will never fully understand life with him around to support her, he leaves and goes back to England - yet he returns several times when things get tough.