Rupert Giles is a Watcher that the Council assigned to work with Buffy after the death of Merrick. He first met Buffy at the school's library where he was working as a librarian. He recognized her as the Slayer and presented her with a book on vampires that she quickly rejected then fleeing the scene. As Buffy wanted to leave her life as a Slayer behind her, she was reluctant to taking Giles as her Watcher, especially with the death of Merrick still fresh in her memory. Nevertheless the Watcher was constant and kept insisting until Buffy finally decided to accept her destiny as the Slayer.

The relationship that Buffy and Giles developed can be seen as both a mentor – student and also as a father – daughter kind of relationship. Giles was supportive of Buffy and aided her not only in her quest as a Slayer but also in her moments of weakness and sorrow, in the hardship of being a teenage anonymous superhero.

Giles' expertise goes far beyond the lore of ancient prophecies and texts regarding the Slayer and the evils she has to face. He's also knowledgeable in certain disciplines of the dark arts, fighting and weaponry.

During his youth he was a bit of a rebellious character, interested in the mischievous chaos magic and he was referred to as “the Ripper” by his friends. He created a bit of a legend about himself. Later he reformed and took upon his task of being a Watcher with the foremost dedication. Yet the misdeeds of his youth came back to haunt him on several occasions. One of his ex friends became a regular threat upon the Scooby Gang, the one Mr. Ethan Rayne. Also on one occasion he was reverted back to his teenage mentality by the doings of this Rayne character, and he ended up having sex with Joyce Summers (Buffy's mother) who was also under the influence of this magic. Buffy didn't find out right away, but when she did, this created a bit of awkwardness between the Slayer and her Watcher.

The Council became annoyed with the way the Slayer’s methods, by involving many other people (the gang) in the process and jeopardizing her secret identity. They decide to enforce a stricter course of action so they destitute Giles as a Watcher and they assign Wesley Wyndam-Pryce as Buffy's new Watcher.

Upon Wesley's incompetence and inability to adapt to the Slayer, Buffy demands of the Council the reinstitution of Giles. Finding themselves in the odd position of being threatened by the Slayer thus diminishing their position of power, the Council reluctantly agrees to reinstate Giles and to tolerate Buffy's methods.

The relationship between Buffy and Giles suffered as the girl experienced many hardships and struggled with life as a Slayer and her “regular” life. Buffy often felt misunderstood by Giles and she began seeking other people's advice and comfort. Giles considered leaving Sunnydale and in fact left for some time. Eventually he returned and remained supportive to Buffy in her quest though they really never seemed to get back on the good terms they used to be.