When he first moved to Sunnydale, Giles was the Sunnydale High Librarian whose sole purpose was to train and guide the recently relocated Buffy in the Slayer ways. His relationship with Buff eventually became a lot deeper then the typical teacher/student relationship between a Watcher and a Slayer. As a matter of fact, it could be insinuated that Giles is actually Buffy's stand-in father figure and certainly feels like she is the daughter he never had. Her real father lives in LA and she hasn't seen him since the dream world was brought to this world and he told her that she was the reason that her parents divorced.

He returned for a short time after her resurection before coming to the conclusion that his presence was hindering her continued growth as a woman and a slayer.

 

He again returned when Willow went over the edge after Tara's murder.  He is the reason that Willow and the world exist today.

Born a Watcher prodigy, Rupert Giles took over Buffy's Slayer training and mentoring when Buffy moved to Sunnydale after her former Watcher, Merrick, had been killed. After attending Oxford University, Giles dreamed of being a fighter pilot or a grocer. Unfortunately, he was forced into the family business of Slayer nurturing. Even after he tried to radically break away from his destiny by becoming the demon summoning Ripper, his intelligence (and the Watchers Council) eventually sent him down the straight road.

Regrettably, the Watchers Council removed Giles because he informed Buffy that he had revoked her powers temporarily so that she could participate in her 18th birthday slayer ritual. The Council, not amused, sent in Wesley Wyndam-Price to take active Watcher duty for Buffy. However, Giles remains in Sunnydale and continues to aid our hero with his vast knowledge. Not a man of many peers, Giles has dated only a few women in Sunnydale including Sunnydale High's computer teacher Jenny Calendar, Joyce Summers and his former girlfriend from England, Olivia.

After quite a long period of being unemployed, Giles took up singing at the Expresso Pump and then bought the Magic Box.  During the war against Glory the Council came with information about Glory but refused to give it to Buffy unless she and her friends all passed their little tests.  When Buffy came to the realization that it was about power, that she had and they wanted, she made them meet her terms which included reinstating Giles as her Watcher with full pay--retroactive to the month they dismissed him.  Shocking everyone in the gang, he returned to England to work things out after Buffy's death and handed the Magic Box over to Anya.