Rupert Giles is a Watcher, assigned by the Watcher's Council to work with Buffy after the death of her first watcher, Merrick. He traveled to Sunnydale, California and took a job as a librarian at Sunnydale High School, where he met Buffy. He recognized her as the Slayer and presented her with a book on vampires, which she rejected, fleeing the scene. With the death of Merrick still fresh in her memory, she was reluctant to take on Giles as her Watcher and to continue her Slayer duties. But Giles was persistent and was finally able to convince Buffy to accept her destiny as the Slayer.
The relationship that developed between Buffy and Giles is primarily a mentor/student relationship, though there are also strong father/daughter undertones. 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.
Rupert Giles was born sometime during the 1950s. He was born into a wealthy family. His family is known to work with the Council Of Watchers for generations. Both of his grandparents were watchers. Even as a child Giles wanted to be like his parents and grandparents. In the BTVS episode "Never Kill A Boy On The First Date" his father told him of his destiny.
As a youth Rupert was very rebellious, which is a source of some ambivalence for him in his adulthood. He dropped out of school, rejecting his Watcher destiny, and took on the nickname "Ripper." He and a group of friends, including Ethan Rayne, Deirdre Page, Philip Henry, Thomas Sutcliff, and Randall (last name unknown), became involved with the dark arts, casting spells and summoning demons for excitement and pleasure. During an attempt to summon the demon Eyghon, Randall became possessed and was eventually killed during the group's attempts to exorcise the demon. After the incident, Rupert broke off ties with these friends and returned to the Watcher's Academy.
Buffy Summers: The Vampire Slayer
At the asking of the Watchers Council Giles goes to Sunnydale, California. As a cover story story he works as the school librarian. His true reasons there are reveled when Buffy Summers(the current slayer) learns of her true destiny. Soon after this he begins to train her as a slayer. He explains to her (and her friends whom have recently been saved by Buffy and know she is a slayer) that Sunnydale sits on top of a Hellmouth. As time goes on they become (as they call them selves and as others call them) The Scooby Gang. While at the school he meets a woman named Jenny Calendar. They eventually fall in love as the scooby gang feels she is the right one for him. But soon as all things bad happen on a hellmouth,Jenny is possesed by a demon who is named Ehgon. The demon is defeated but Jenny feels that the reason why she was possessed is because of Giles and his association with the slayer. She decides that she needs time to sort out what she really wants taking a break from the relationship.much to Gile's dismay. Eventually Jenny is revealed as a member of the gypsy clan who cursed Angel centuries ago.Giles feels angry because she did not tell him earlier in their relationship. Unfortunately they never got to fully reconcile, as a soulless Angel wreaks havoc of Buffy and her friends. He next targets Giles,he comes to the school where he murders Miss Calendar as she is finishing the spell which will return his soul to him, however Angelus catches her as she flees snapping her neck. Shortly after this an unsuspecting Giles comes home where he is surprised to find red roses, fine wine,music and a note telling he to go up stairs. As he approaches his room he sees Jenny on his bed, but sadly,as he approaches he realizes to his horror it to only be her corpse, nothing but a cruel trick by the soulless Angel. Giles then seeks vengeance on Angelus, despite Buffy and the rest of Scooby Gang's advice it would be suicide. However he does not listen as he later burns down Angel's home while he is inside. Later in the series the gang catches a glimpse of Gileses' "Ripper" days. After he eats a cursed candy bar he reverts (mentally) to is youth. During this time he has sex with the mother of Buffy mother Joyce Summers.
Since his return from hell Angel was kept a secret from him and the gang by Buffy. When Giles finds out that Buffy is having a relationship with the man that killed his only lover in years (Jenny) he feels betrayed. But after giving it serious time and thought he realizes that Angel now has a soul and is a different man than the one who killed Jenny. So he agrees to forgive Angel.
The Magic Box
After he is fired from the Watchers Council. Giles no longer enjoyed the unemployment life so he decide to buy a small magic shop( the original owner was eaten by vampires). As Buffy had grew in knowledge and skill. She didn't need Giles as much anymore so he felt like he was no longer needed. When Ethan Rayne casts a spell on Giles it turns him into a demon (episode "A New Man") he can no longer peak any English and is unable to communicate with Buffy and the scoobies. Luckily Spike can understand him and uses him to communicate. Unfortunately Buffy doesn't know that Giles IS the demon but think that the demon ate Giles. She kills him, but while she has the weapon in him she sees that he has those classic Giles eyes of "O brother" and Buffy realizes that the demon is Giles. Ethan is taken into containment by the initiative.
Back to England and Back to Sunnydale
Months after the death of Buffy Summers Giles decides that he wants to return to England. The same day he leaves the Gang (Willow, Xander, Anya, and Tara) bring Buffy back to life. After he returns he learns that the team brought Buffy back to life. Even though he is thrilled to have Buffy back he is angered with the entire scooby gang especially Willow for using such dark magics. He calls her a "rank, arrogant amateur!" Their relationship is strained to the point that Willow threatens him. The Gang,who believed that Buffy was in hell, who was actually in Heaven revealed in "Once More, with Feeling". After the shocking death of Buffy's mother Joyce, Buffy begins to rekindle her father-daughter relationship. But as Giles notices her increased dependence on him for personal reasons other than slayer reasons. He fills that Buffy needs to grow up and he feels that him being their is keeping her from being the strong women that she can be. So he gives the Magic Box to Anya and moves back to England and begins to work with a coven.
Months after Giles left Sunnydale Tara (Willow's girlfriend) is shot by a stray bullet. This drives Willow over the edge as she was already abusing magic. But after Tara was shot by Warren Mears she turns to the dark side of magic. She seeks revenge on Warren and his friends Jonathan Levinson and Andrew Wells. She kills Warren but before she can Kill his other two friends Giles attacks her with the power of the coven he was working with. She laughs at his attempts. After a short battle Willow overcomes him and steals the magic he was given. But Giles let Willow take his Magic because he has supernaturally "drugged" it so that it would tap into the tiniest bit of humanity.
After Willow is brought back to the side of good she and Giles return to England for her to enter a wiccan rehab.
The Final Battle
Months after "Dark Willow Saga" Giles returns to Sunnydale and bring new to Buffy about the very first evil called "The First Evil" and its demonic legions the most common are the bringers. Giles reveal that he stole volumes from the watchers HQ before it was destroyed by an agent of the first called "Caleb" this gives them information about a weapon of great strength. A weapon meant for the slayer, a scythe. During the battle Willow uses power magic to give each potential slayer in the world the full abilities of a slayer. After the battle is over the entire gang looks at the giant creator that was left after the collapse of the hellmouth and thinks about the future.
Buffy Season 8
After the Slayer's final battle against the First Evil, the Slayers have become a global organization fighting vampires and demons throughout the world, with Giles being in charge of one of the squads of Slayers. During this time he seeks assistance from the vampire slayer Faith, in order to assassinate a rogue slayer Genevieve Savidge, before she could enact her plan of assassinating Buffy. Following Genevieve's assassination, Giles and Faith break away from the Slayer's organization, to venture out on there own in order to stop Slayers from going down the path of evil, but because of his working with Faith Giles has a falling out with Buffy, leaving them on non speaking terms.
Shortly into his partnership with Faith, they begin investigating rumors of a slayer's sanctuary. Once they reach the town, Giles runs into a former watcher by the name of Duncan Fillworthe, who tells them vampires don't enter the town of fear of the Slayer army in the town. On learning this, Giles tries to argue that a stalemate is not the solution to dusting vampires. Duncan then reveals the slayers haven't been protecting the town, they are being fed to an ancient demon, learning this Giles frantically tries to warn Faith of the danger, while Duncan tries to convince Giles that the slayers each deserve this fate for his believing that they betrayed the Watchers Council, and trying to convince Giles that Buffy deserves this more then any other slayer, even bringing up that her relationship with Angel ultimately led his girlfriends Jenny Calenders death. Giles ultimately finds Faith already battling the demon, which itself becomes stronger with his enemies regrets and fears. In his attempts to free the young vampire slayer Courtney, he begins a fight with Duncan, which during this the demon devours Duncan, allowing Giles to save Courtney, and Faith defeats the demon. With the demon dead, and the slayer's having been devoured, Giles and Faith rally the townspeople to defend themselves against the oncoming vampires, that are attempting to invade.
After Harmony turns the public's perception of vampires favorable, and having the world turn against the Slayers. Giles and Faith end up forced into hiding in a bunker, but when two demons find them they ultimately retreat to Scotland where they join back up with Buffy and the gang. During this rendezvous, Giles and Buffy reconcile their differences, before being transported into the Tibet where they regroup with fellow Sunnydale Alum Oz, who teaches the slayer's how to suppress their magic which stops Twilight from being able to track them.
It was during this time that Giles revealed the reason he had been traveling around the world was to find a weapon capable of killing the Slayer who would eventually be the recipient of immense powers, and be the creator of a new world. While this was believed to be nothing but a watchers myth, Giles knew it would be Buffy.
During the battle for Sunnydale, Giles attempts to use the scythe, to destroy the seed of wonder. When he attempts to this, the Twilight possessed Angel stop him, by snapping his neck, resulting in his death. After his death, Buffy managed to destroy the seed, which resulted in the end of magic on Earth. In Giles will, he left everything to Faith, so she could be free of violence that had followed her entire life. While to Buffy he left her the vampyr tome that he had shown her during their first meeting, and a note telling her that he always considered her the one true vampire slayer.
Powers and Skills
Giles has a very large knowledge of demonology and Slayer combat all from teachings while at the watchers council. Although he has no "true" supernatural powers he can deal with most threats (demons,vampires,etc..) just as well if not better than Willow or Buffy. Because of his "Ripper" days he can control some magic (but not at the level of someone like Willow). Giles knows several different languages Latin, German, Greek, Japanese, Sumerian, Gaelic, Cantonese and Mandarin. Giles is also trained in hand to hand combat, and is skilled enough to fend off several vampires. But Giles prefers to let Buffy do the physical aspects of slaying.
Giles body can become a vessel of power magics. During "Dark Willow" he was granted teleportaion, powerful energy blast, and telekinesis that rivaled Willow's. These powers were granted to him by the powerful "Devon Coven."