Rupert Giles is a very British watcher. He knew he was going to be a watcher by the time he was 10, and rebelled by leaving Oxford and dabbling in the black arts. He eventually came to his senses and waited to be called amongst the collections of the British museum.
He was sent to Sunnydale to be Buffy's watcher and arrived before she got there. Buffy first came with a negative attitude to slaying, but Giles set her right. He supported Buffy all through the year, and was upset when he discovered a prophecy telling him that Buffy would face the master and die. She was upset when she found out, and Giles then said he would get ready to face the master himself. But Buffy took back her role as slayer and knocked him out in the process.
There are more new surprises round the corner too; Giles dates Jenny Calendar, a computer-science teacher and witch. He doesn't know it but she was sent to Sunnydale by her gypsy tribe to keep watch on Angel. The two flirt, and their first date was a football rally, not Giles's perfect choice, but the two are well suited. Even Xander and Buffy give Giles tips on how to date her.
Things rarely go smoothly...an old friend of Giles's, Ethan Rayne, comes to town. It turns out that Giles is not at strict and book-loving as the gang think, in his teenage days he and his friends released a demon and had it's mark tattooed on them. The demon comes to Sunnydale and starts its rampaging, possessing Jenny Calendar before Giles can kill it, and their relationship is put on hold.
The gang hold a surprise party for Buffy's birthday, and Giles brings along his date. When Buffy sleeps with Angel and he turns evil, Giles is adamant for Buffy not to blame herself, but says she did act on a whim, and should have thought first.
His relationship with Jenny blossoms but sadly Angelus kills Jenny, he snaps her neck for trying to curse him with a soul again. Giles had arranged for Jenny to come over later that night, but Angelus gets there first and lays a trap for Giles; lining the stairs with roses and laying Jenny's body in his bed. Giles goes into full 'ripper' mode and attacks Angelus, managing to do some damage with fire torches before Angelus gets the upper hand, and Buffy eventually saves Giles. She pulls him out into the street crying, and Jenny's funeral is later that episode.
Season 3 Buffy has to take her 'crucentium', a test that all slayers face at 18 by order of the watcher's council. Giles betrays her, but then when the demon she has to face escapes and threatens to attack her mother, Giles cheats on the test, and helps her. For doing that, Buffy passes, but Giles is fired as watcher, and Wesley Wyndam-Price appears on the scene, briefly staying as Buffy and Faith's watcher. But Buffy quits the council for the refusal to cure Angel and Giles is back to his rightful position as watcher. Wesley leaves for L.A.
Season 4 comes, and Giles is a 'gentleman of leisure' with no library to run. But he still has his books, and his house becomes the general meeting place for the scoobies. In the episode 'Hush' Giles's new girlfriend, Olivia, comes to town, stays with Giles for a few days, and helps defeat the gentlemen, despite not knowing what's going on.
There are more people visiting Giles's apartment too, with the arrival of Spike-with-a-chip-in-his-head, chained in Giles's bathtub, drinking pig's blood out of a mug which reads 'kiss the librarian', until they can get information out of him and make sure he's harmless.
At the end of the season Giles joins with Willow and Xander to imbune Buffy with their powers. The outcome of the spell is that Buffy defeats Adam, but the first slayer visits them in their dreams, tries to kill them all, and leaves cryptic messages about the future. In the joining, Giles is the brain, or intellect.
Since Giles has no library to run, he has no job. At the beginning of season 5 he is bored of being a slacker type (which is valid, lifestyle-wise) and buys the local magic shop. (The previous owner was murdered.)
In the episode 'Buffy vs. dracula' Giles is seriously contemplating returning to England for good, but doesn't, when Buffy tells him she needs him to be her watcher again. He stays, and is glad he did when Buffy's mother dies, and he needs to be the strong father figure to Buffy, Dawn and the scoobies. There's a close moment when Giles is shot with an arrow and has a severe chest wound, until Buffy calls Ben to give him medicine. The gang head back to Sunnydale and Giles is the one who finally stops Glory, Buffy not being able to kill a human, but Giles snaps Ben's neck, killing Glory. Everyone is devastated when Buffy dives off the bridge to save her sister and the whole world. With no slayer to watch, Giles retreats to England.
But he comes back again. Hearing that Willow has resurrected Buffy and is none to happy about it, Giles shouts at Willow for her abuse of magic, and goes back to training the slayer. In 'Once More with Feeling' he sings about standing in Buffy's way so she can't grow up as an adult, but Buffy remains oblivious. In the next ep, Willow does a spell to make them all forget who they are, and there are some hilarious scenes between Giles and Spike, who assume they're father and son, both being British, and assumes Anya is the mother. This results in a passionate kiss between Anya and Giles before he leaves for England.
But, even when Buffy sings that she was in heaven, he has to leave for England to stay for good. And there he does until he hears from a coven in England, that Willow has been further into her abuse of magic and plans to kill Jonathan and Andrew as well as Warren. He teleports himself to the Magic Box using borrowed magicks just in time to break apart the Slayer / Witch showdown. He traps Willow in a frozen state for about 2 hours, so he can catch up with Buffy and Anya. They laugh and joke about the dark things that have been happening in Sunnydale. That is, until, Willow breaks free of her frozen prison, hypnotises Anya, blows apart the magic box and starts torturing Giles. He knew she would take his borrowed magic, and she does; controlling the essence of pure magic, which proves too much for her, she can feel all the world's pain. Deciding that it is too much, she plans to destroy the world and destroy the pain. Giles sends Xander to stop her, being her oldest friend and knowing the most about her. Xander manages to break through to her and tells her he loves her.
Willow is be taken to a rehabilitation centre in England to be cared for by Giles at the start of season 7.