Although the Scooby Gang later joked that he wore tweed diapers as a child, Giles was in fact a rebellious youth, rejecting his responsibility as a Watcher and dropping out of Oxford University, where he was studying history, when he was twenty-one. During this time, Giles claimed to be a founding member of Pink Floyd in order to impress girls, and may have delved into criminal activity (he remarks that hot-wiring a car is "like riding a bloody bicycle"). He is a talented singer and guitar player (as Head is in real life) which the gang discovered, to their horror, when they saw him singing "Behind Blue Eyes" by The Who in "Where the Wild Things Are". Spike also saw him singing "Free Bird" by Lynyrd Skynyrd in "The Yoko Factor".
He began to explore dark magics and befriended a group of young people who delved into the dark arts for fun or money: Ethan Rayne, Philip Henry, Dierdre Page, Thomas Sutcliff, and Randall. Giles gained the nickname "Ripper" during this time. Together, the group summoned a particularly grotesque demon called Eyghon, who would eventually murder Randall. Following Randall's death, Ethan and the others failed to exorcise Eyghon, and Giles accepted his destiny of becoming a Watcher. Before becoming a fully-fledged Watcher, he also worked as "the curator of a British museum, maybe the British Museum" as Willow says ("Welcome to the Hellmouth").
As the Watcher, librarian, and general authority figure, Giles often delivers exposition. He is a father figure to Buffy and an advisor to her friends Xander Harris and Willow Rosenberg, together forming the "core four" of the Scooby Gang. Giles is often portrayed as somewhat of a "straight man" and his "stuffy" Oxford sensibility serves as counterpoint to the stereotypical Southern Californian characters and setting. He makes a "weird cluck-cluck sound with his tongue" when he is angry, but "too English to say anything" ("Faith, Hope & Trick"). He admits to being technophobic, a fact which often brings him into conflict with technopagan and computer science teacher, Jenny Calendar. However, after Jenny aids him in casting the demon Moloch out of the Internet, the pair reach an understanding and begin a romantic relationship.
In Season Two, Giles' dark side is revealed and his relationship with Jenny deepens. In "The Dark Age", Ethan Rayne comes to Sunnydale to flee the demon Eyghon. Giles ashamedly admits to Buffy that he was responsible for summoning the demon in his youth, and is horrified when Jenny becomes possessed by Eyghon. Although Eyghon is defeated, Jenny takes time to cope with her ordeal, retaining distance from Giles in the process. When new Slayer Kendra Young arrives in Sunnydale, Giles shares with her an appreciation for obscure texts, resulting in Buffy nicknaming her the "She-Giles." Giles feels betrayed when Jenny reveals she is actually a member of the Kalderash gypsy clan, sent to keep an eye on Buffy's relationship with the vampire Angel. When Angel loses his soul and murders Jenny, leaving her corpse for Giles to find in his apartment ("Passion"), Giles seeks revenge by burning down Angelus' home. Angelus later kidnaps and tortures him, and Drusilla hypnotizes him into thinking she is Jenny, so he will reveal how to awake Acathla. Buffy is forced to kill Angel, despite Willow restoring his soul, and subsequently leaves Sunnydale.
In Season Three, the paternal relationship which Giles feels for Buffy is strengthened significantly. He spends the summer desperately following up any clues of Buffy's whereabouts, and is overjoyed when she finally returns months later. Giles briefly serves as Watcher for Kendra's replacement Slayer, Faith Lehane. The Scoobies are given another disturbing glimpse in Giles' past when, along with every other adult in Sunnydale, he is reverted to a teenager by enchanted band candy supplied by Ethan Rayne. During this time, he indulges in theft and vandalism, and has sex with Buffy's mother, Joyce Summers. When Buffy keeps Angel's return from hell a secret from the other Scoobies, Giles feels betrayed by her affection for the man who tortured him and murdered Jenny, but later agrees to help Angel in "Amends".
As Buffy's Cruciamentum approaches, a brutal tradition of the Watchers' Council in which a depowered Slayer is forced to battle a vampire, Giles struggles to cope with the guilt of betraying Buffy's trust. Despite describing the test as "an archaic exercise in cruelty", he secretly strips Buffy of her powers, before eventually coming clean when the vampire she is meant to fight escapes. Buffy is disgusted, but is later moved when Head Watcher Quentin Travers dismisses Giles for having a father's love for her. Giles is fired, and replaced with Wesley Wyndam-Pryce, but continues to act as Buffy's unofficial Watcher. When Buffy briefly finds herself endowed with telepathic powers, she discovers that her mother believes Giles to be "like a stevedore in bed." In a battle with the demonic Mayor of Sunnydale, Giles presses the trigger which destroys the Mayor and Sunnydale High, putting himself out of a job.
In Season Four, Giles must cope with being unemployed, and the growing knowledge that Buffy no longer needs him. He continues a sexual relationship with his old friend Olivia, who Anya tactlessly labels his "orgasm friend." Lacking a sense of purpose, he spends most of his time lounging around his apartment, watching Passions with Spike. He begins to suffer from depression, especially when the Scoobies fail to keep him in the loop regarding Buffy's new boyfriend Riley Finn and his membership in the Initiative. In the episode "Something Blue", Giles becomes blind as a result of a faulty spell cast by Willow Rosenberg. When Ethan Rayne casts a spell on Giles which turns him into a Fyarl demon ("A New Man"), he must enlist Spike's help to escape the Initiative and Buffy, who believe him to be a demon who murdered Giles. However, Buffy later realizes from his eyes and "annoyed" expression that he is in fact Giles, and Ethan is taken into custody by the Initiative. In order to defeat the cyber-demonoid Adam, Buffy, Willow, Xander and Giles cast a spell to combine their strength. Giles provides the "mind", and Buffy is able to defeat Adam.
In Season Five, Giles finds a new purpose as Buffy embraces her destiny as the Slayer. When the owner of The Magic Box is killed by vampires, Giles is convinced by the shop's high profit margins to buy it, hiring Anya as his overtly enthusiastic assistant. Buffy discovers that her sister, Dawn Summers, is actually the Key; mystical energy in human form to protect it from the hell-god Glory. Buffy initially confides only in Giles about Dawn's true nature, and he decides to contact the Watchers' Council for more information about Glory. In "Checkpoint", Quentin Travers blackmails Buffy into obeying the Council's demands, threatening to have Giles deported if she does not comply. Buffy stands up to the Council, informing them that they will release all information concerning Glory to them, and will re-instate Giles as her official Watcher, to be paid retroactively from the month he was fired. Travers reluctantly agrees, and leaves.
As the Scoobies labor to find a way to defeat Glory, Giles brings up the difficult idea of killing Dawn to end her plans. Buffy refuses to listen, and vows to protect Dawn at all costs, much to Giles' anger. Giles admits that he loves Dawn, but that sacrifices have to be made. It is revealed that Glory shares her body with an innocent human male called Ben, and can be killed if Ben dies. In the final battle against Glory, Buffy decides to spare Ben's life, but Giles is less merciful. Explaining that Buffy is a hero and therefore different from them, he suffocates Ben with his bare hands ("The Gift"). Buffy later sacrifices her own life to save Dawn's, saving the world in the process.
Season Six sees Giles reluctantly stepping back to allow Buffy to gain independence. Several months after her death, Giles decides to return to England. On the very day he leaves, Willow, Xander, Anya and Tara resurrect Buffy, and he comes back as soon as he hears of this. Despite being overjoyed to have Buffy back, he is furious at Willow for invoking such dark magic, and angrily dismisses her as "a rank, arrogant amateur." As Buffy begins to over-rely on Giles for financial and emotional support, he decides that his presence is preventing her assuming responsibility for her own life. He leaves again for his native England, moving to a place near Bath, where he works with a powerful local coven.
A few months later, Tara is killed by a stray bullet as Warren Mears attacks Buffy. Willow, still recovering from an addiction to dark magic, suffers a relapse, kills Warren, and attempts to kill his former partners in crime, before resolving to end humanity's pain (and her own) by destroying the world. Hearing about a dark power rising in Sunnydale, Giles teleports back there, wielding great magical power borrowed from the Devon Coven. As Dark Willow boasts of her indestructibility, Giles knocks her to the floor with a blast of magic energy, stating "I'd like to test that theory" ("Two to Go"). Knowing that Willow is too strong, he tricks her into draining him of his magics, which brings him near death. It also allows Xander to reason with Willow as the good magic brings out her natural love and compassion, eating away at the evil within her. Giles returns to England with Willow for her rehabilitation. A few months later, he brings Potential Slayers to Sunnydale to protect them from the First Evil and its Bringers. Giles had removed a few volumes from the headquarters of the Watchers' Council, which is soon afterward destroyed by Caleb, an agent of the First. An injured Watcher named Robson witnessed Giles about to be decapitated by a Bringer, before blacking out. When the Scoobies hear about this, they worry that Giles may have been killed, and that the First is merely impersonating him. They are relieved, however, when they manage to tackle him to the ground, proving he is corporeal and therefore not the First. Giles later loses Buffy's trust somewhat when he takes part in a scheme with Robin Wood to kill Spike ("Lies My Parents Told Me"), with Buffy telling him, "I think you've taught me everything I need to know." Ironically, Giles was proved to be correct in his belief that Buffy kept Spike around out for personal reasons rather than tactical ones when she talks to Spike the night before the final battle under the Hellmouth. In the series finale, he is present at the battle of the Hellmouth, and survives.
In Season Eight, the Scooby Gang has expanded into a global organization, training approximately five hundred Slayers spread over ten squads. Giles is head of the Slayer operations in England, parallel with Andrew's operations in Italy and Buffy and Xander's in Scotland, and he keeps in close contact with Xander and Andrew, discussing issues. In the Season Eight arc, "No Future For You", he comes to Faith to request her assistance assassinating another Slayer, who will bring around the Apocalypse if left unchecked. After they are both forced to make the difficult decisions to kill the rogue Slayer Gigi and her mentor Roden, they decide that they will work together in the future as equal partners, keeping Slayers from turning down the dark path both Faith and Gigi walked.
Shortly after Buffy ended its seven year televised run, there was talk of a Giles-based spin-off series for the BBC entitled Ripper. As of Comicon 2007, Joss Whedon has confirmed Ripper is still planned and in the pipeline.
Giles has moderate skill in hand-to-hand combat, as well as various melee weapons. While his demeanor is typically mild and polite, Giles is not above using raw violence to solve a problem, such as physically threatening Principal Snyder into readmitting Buffy to school after her expulsion, pummeling Angelus senseless with a flaming baseball bat and then attempting to set him on fire upon discovering that he had killed Jenny Calendar, suffocating Ben, Glory's human host, with his bare hands, and severely beating up Ethan Rayne for information (or simply to make himself feel better). Typically however, Giles' calm demeanor and professionalism offer him a detached state of authority even in the face of fearsome monsters, as demonstrated during his confrontation with a violent demon in "The Long Way Home". He is also shown to be able to quickly hotwire a car in the episode "Dead Man's Party".
In the Season Six finale, Giles was temporarily granted with the powers of the Devon Coven in an attempt to stop Willow. During this time, he demonstrated powers such as teleportation, telekinesis, and energy projection. He lost all of these powers when Willow drained him and left him on the brink of death, but he recovered immediately after Xander calms her down from her rampage.