Gwen Elizabeth Cooper is a fictional character portrayed by Welsh actress Eve Myles in the BBC science-fiction television programme Torchwood, a spin-off of the long-running series Doctor Who. The lead female character, Gwen featured in every episode of the show's 2006–2011 run, as well as two 2008 crossover episodes of Doctor Who. Gwen appears in Expanded Universe material such as the Torchwood novels and audiobooks, comic books and radio plays.

Within the series narrative, Gwen is a South Wales Police officer who discovers the mysterious Torchwood Institute, into which she is recruited by Jack Harkness (John Barrowman). Gwen initially operates as a field agent, bringing a humane and rounded approach to the team's investigations of the extraterrestrial. Gwen's work begins to take a toll on her personal life in the first series (2006), but her marriage to Rhys (Kai Owen) keeps her grounded, even as her responsibilities increase in Series 2 (2008). In Children of Earth (2009) and Miracle Day (2011), after Torchwood is destroyed to conceal a government conspiracy, a much hardened Gwen operates under her own mandate as the world undergoes crises linked with unprecedented alien threats.

Gwen is introduced as an audience surrogate, in the mould of the "girl next door" archetype, much like the perennial "companion" character in Doctor Who. However, as the series progressed, the production team chose to emphasise contradictory aspects to her character by having Gwen make ethically dubious decisions. As the show progresses, and even after becoming a mother, Gwen evolves into a more militant action heroine and finds herself willing to make tough decisions to protect those closest to her.

Reviewers have generally responded positively to the complexity of Gwen's character and Eve Myles' portrayal, though critics found fault with her more prosaic characterisation earlier on. Eve Myles' portrayal of Gwen has been cited as effective in both promoting the country of Wales and combating a perceived prejudice against the Welsh accent. Myles received a Welsh BAFTA for the first series of the show and was nominated for several other acting awards across each of Torchwood's four series'.

Gwen is introduced in the first episode of Torchwood, in which she—a Cardiff police officer— witnesses Captain Jack Harkness (John Barrowman) and his team interrogating a dead man in an alley. After investigating the Torchwood team, she locates the Torchwood Institute's Cardiff branch, home to a team of alien hunters. Though Jack wipes her memories, they are later triggered by reports of murders committed with exotic weaponry. Torchwood's own Suzie Costello (Indira Varma) is the serial killer. Gwen is present at Suzie's attempt to kill Jack, and Suzie's ensuing suicide. Jack reveals that he cannot die and offers Gwen a job in Torchwood. Jack and Gwen's relationship takes on a romantic hue, despite her long-standing relationship with boyfriend Rhys (Kai Owen). Gwen falls into a short-lived destructive affair with colleague Owen (Burn Gorman), after a particularly traumatic experience with cannibals, which she cannot share with Rhys. Shortly afterwards, Gwen is targeted and almost killed by a vengeful undead Suzie. In their conversations, Gwen is shocked to realise the extent to which Torchwood neglected Suzie's emotional welfare, and Suzie is resentful towards Gwen, who has replaced her both professionally, and as Owen's lover. Wracked with guilt, Gwen confesses her affair with Owen after it ends in "Combat", but she drugs Rhys with an amnesia pill so he will not remember her confession. In the series one finale, her desire to resurrect a murdered Rhys motivates her to mutiny against Jack by opening the rift in time and space which Torchwood monitors. After Jack sacrifices himself to the released supernatural demon, Gwen maintains a vigil, certain that he will resurrect. As she loses hope, giving Jack a parting kiss, he awakens and thanks her. Shortly afterwards, Jack abandons Torchwood to reunite with the mysterious "Doctor" from his past.

In 2008, as series two begins, Gwen has replaced Jack as team leader. When Jack returns to lead Torchwood, Gwen displays anger towards him for abandoning the team, and surprises him with the news she has become engaged. In a later episode she is forced to tell Rhys about her job at Torchwood when he assumes she is having an affair with Jack. After Rhys takes a bullet for her Gwen defies Jack by refusing to wipe Rhys' memories, feeling humbled by her partner's loyalty and bravery. In "Something Borrowed", despite being pregnant to an alien Nostrovite, Gwen refuses to postpone the wedding to Rhys. Her marriage makes Jack remember an earlier marriage of his own. In "Adrift" Rhys continues to push Gwen to unburden herself from her traumatic experiences, after her attempt to re-unite families with missing children stolen by the rift goes horribly wrong. In the series finale, Gwen's leadership skills allow her to co-ordinate Cardiff during the chaos caused by Captain John Hart (James Marsters) and Jack's younger brother Gray (Lachlan Nieboer). When Gray kills Gwen's teammates Toshiko (Naoko Mori) and Owen, Gwen is desolate. The character makes a crossover appearance in Doctor Who alongside Jack and Ianto (Gareth David-Lloyd) in its 2008 series' two-part finale, in which Torchwood help contact series protagonist the Doctor (David Tennant) during an invasion of the psychopathic mutant Daleks, and later must defend their headquarters from the attacking aliens. The Doctor suggests a connection to Gwyneth, a separate character portrayed by Eve Myles in the 2005 Doctor Who episode "The Unquiet Dead". She was also mentioned in the alternate universe created in the Doctor Who episode "Turn Left" when Rose Tyler (Billie Piper) mentioned that she and Ianto were killed in the attempt to stop the Sontarans' invasion of Earth with ATMOs devices, whilst Jack is imprisoned on their homeworld.

In Children of Earth, a five-part serial broadcast in 2009, Gwen is a more militant heroine shaped by the increased responsibilities and pressures of her job. In part one, aliens called the 4-5-6 communicate to the world by taking possession of its children and a middle-aged man, Clem McDonald (Paul Copley). Gwen investigates by visiting Clem, who—being possessed of psychic abilities—announces to her she is pregnant, which she later confirms using technology at the Torchwood Hub. Gwen is forced to defend herself from assassins after the Hub is destroyed to conceal a government conspiracy, taking Rhys on the run with her, and re-establishing Torchwood Three in a London warehouse. By making contact with government PA Lois Habiba (Cush Jumbo), Gwen discovers her fugitive status and is able to survey cabinet meetings. When Ianto is killed fighting the 4-5-6, Gwen and Rhys return to Cardiff to protect his niece and nephew from the armed forces, ordered by the government to sacrifice a percentage of the country's children as appeasement. She feels she now understands the Doctor's periodic absence during various crises in Earth's history; believing him to sometimes "look at this planet, and turn away in shame". In the dénouement of the final part, Gwen is six months pregnant when she returns Jack's vortex manipulator to him. She insists he stay; he leaves her behind and abandons Earth.

Fourth series Miracle Day (2011), an American co-production, re-establishes Gwen, Rhys and their daughter Anwen living reclusively in rural Wales. CIA agent Rex Matheson (Mekhi Phifer) renditions Gwen to the United States, believing a worldwide phenomenon wherein no one can die to be connected to Torchwood and she becomes part of a new Torchwood team alongside Jack, Rex and former CIA analyst Esther Drummond (Alexa Havins). In "The Categories of Life" and "The Middle Men" Gwen returns to Wales to rescue her ailing father Geraint (William Thomas), who survived a fatal heart attack on "Miracle Day", from a facility where the critically injured are incinerated. Upon her return to America her family are kidnapped as leverage to make her surrender Jack. Gwen tells Jack, now ostensibly mortal, that she would allow him to be killed to save Anwen. After Gwen is deported by senior CIA official Allen Shapiro (John De Lancie) because of her dissidence she focusses on protecting her father from the authorities; enclosing him in her mother Mary's (Sharon Morgan) cellar and supplying him with palliative medicine. The concluding episode, "The Blood Line" opens with Gwen eulogising her father as the kindest man she has ever known, before stating that she is about to kill him. When the power and profit driven conspirators behind Miracle Day try to thwart her resolve by wounding Esther, Gwen declares she will sacrifice her entire Torchwood team to restore order. She shoots Jack through the heart, allowing his unique blood to prompt an ancient phenomenon to reset the human morphic field. As Geraint and other "Miracle Day" survivors pass away peacefully, Jack's immortality is restored. Gwen is shocked to discover at Esther's funeral that Rex is now immortal too.