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.
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. You see, her real father lives in LA and she rarely sees him any more.
Regrettably, the Watchers Council canned Giles because he informed Buffy that he had to revoke her powers temporarily so that she could participate in her 18th birthday ritual. The Council, not amused, sent in a new Watcher named 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 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.
It seems as if the loss of Buffy has affected Giles in a way that it would if she was his real daughter. Shocking everyone in the gang, he is returning to England to work things out and handing over the Magic Box to Anya.