Captain Martin Walker is the protagonist and player character of the 2012 third-person shooter video game Spec Ops: The Line, the seventh entry of the Spec Ops series, developed by German studio Yager Development and published by 2K Games. Walker is a member of Delta Force, a special operations force of the United States Army. During the events of The Line, Walker is tasked with leading a team on a recon mission to a disaster-ravaged Dubai. Sometime after his arrival in Dubai, Walker decides to search for his former superior, Lieutenant Colonel John Konrad, who is last known to be leading the city's evacuation and relief efforts. He begins committing unethical acts of violence against other survivors in the city, and experiences hallucinations which distort his perception of the game's subsequent events.

Walker is designed to be the protagonist of a video game experience which challenges the player emotionally through its exploration of discomforting themes associated with the efficacies and consequences of war on its combatants. Lead writer Walt Williams was largely responsible for Walker's characterization, which underwent changes in tandem with the revisions to the story of The Line during its development. Williams eventually decided that Walker's sanity would gradually deteriorate as the story progresses. He believed that this provides greater narrative depth as Walker's experiences explore the extent of a soldier’s humanity and the impact of his psyche from the difficult choices he has to make.

Voiced by American actor Nolan North, Walker is noted by critics for his distinct characterization among protagonist characters in the video game industry, as The Line focuses on exploring the ethical implications of the player's direction of Walker's decisions as part of the game's storyline. Walker's motivations have been widely discussed by commentators, who explore and analyze the series' underlying themes in various publications and academic writings.

In an interview with GamesRadar, lead writer Walt Williams explained that the development team of Spec Ops: The Line desired to do a "dramatic, more raw war story", something that no other entity in the video game industry was attempting at the time. The story arc of its protagonist, Captain Martin Walker, is written to subvert the military shooter genre in a number of ways. In particular, it undermines the notion that intervention from the United States military is based on virtue; challenges the portrayal of war in popular media which is often inconspicuously devoid of civilian casualties; highlights the traumatic effect of war on surviving soldiers; and exposes the ambiguities between battlefield friends and enemies.

The character's surname Walker was chosen because he would end up traversing much of the game's levels by foot. Walker is initially presented with a personality that evokes a virtuous military serviceman archetype: clean cut, courteous to his colleagues, and respectful of the rules of military engagement. The writing for the camaraderie between Walker and his squad mates was inspired by HBO's Generation Kill. As every motivation Walker has in the game is designed to mirror the motivation of someone who plays a video game, his emotional state is often explicitly reflected to the player. Because the player and Walker is intended to be the same person according to the game's design philosophy, the character is presented as an "average guy" who is relatable to the player in spite of his special forces training background. Much of his personality is expressed through his body language and his reactions during battle sequences. Lead designer Cory Davis described the team's attempt behind such a characterization to be "tough", as they wanted to express as strongly as possible how Walker would evolve from a "blank canvas" to a psychologically damaged person after experiencing some calamitous events within the story. Walker’s appearance and demeanor grows more ragged or unhinged the closer he gets to his target and is indicative of the toll it has taken on his sanity following hours of intense firefights. The 1990 film Jacob's Ladder provides a reference point in characterizing Walker's post-war traumatic experience.

Williams characterized Walker's main flaw to be his inability or unwillingness to cut his losses and let go of his mission even though it is doomed to fail. Instead, Walker would double down on his hunt for Colonel John Konrad, Walker's former superior during their time serving in Afghanistan, and is adamant in his belief that the risk surrounding his attempt to apprehend the person supposedly responsible for Dubai's disastrous conditions will eventually pay off. Walker uses lethal force not because he wants to defeat Konrad, but because he is determined to convince himself that he is the hero of the story; his desire to be a "good man" and his misguided conviction that every bullet he fires makes the world a better place instead results in further ruin and senseless loss of life around him. In his book Significant Zero: Heroes, Villains, and the Fight for Art and Soul in Video Games, Williams revealed that Walker was originally supposed to pursue a still-living Konrad to the top floor of Dubai's tallest skyscraper. This follows an extended period of time where Konrad serves as a major presence by antagonizing Walker via radio broadcasts throughout the story. At one point during the development of The Line, Williams decided to reframe the story. He introduced a plot twist which takes place by the end of The Line, which reveals that Konrad was already dead prior to the arrival of Walker's squad in Dubai and that he was hallucinating Konrad's messages to him the entire time.

According to Williams, much of the game's events are intentionally left open to interpretation. From his point of view, Walker died in a helicopter crash at the end of the game's prologue and that the entirety of its narrative afterwards exists in a state of purgatory, hallucinated by Walker as he lay dying in the wreckage. The prologue sequence itself, which begins in medias res with a helicopter chase involving Walker's squad, was mandated by senior management overseeing the development team. In response, Williams decided to discreetly alter the context of the game's story by asking the characters' voice actors to re-record a few alternate lines for the sequence, which shows Walker experiencing a déjà vu moment later in the story. Williams explained that Walker's unstable psychological state is due in part to his denial over the atrocities he unwittingly committed against innocent civilians earlier in the narrative.

Nolan North was chosen to voice Walker. To North, "a good story starts with a good script" and that he would only consider improvising or deviating from the script whenever he is convinced that the improvisation is objectively an improvement over the pre-determined material. For his performance, North mostly adhered to the script for The Line as he found it to be consistently well written with an engaging story. However, North was unaware that he was directed to make a déjà vu reference when he was asked by Williams to re-record the lines for the helicopter sequence. North is personally acquainted with career military people and noted in interviews that the role is not an unusual one for him. In the beginning, he was given a description of Walker's backstory, which included details explaining who he is, where he comes from and where he grew up. To North, The Line possesses a hidden depth that is uncommon within the shooter game genre. For example, Walker's backstory humanizes him as he is essentially a man in a uniform, who would have otherwise led a typical civilian life if he is not serving in the military like many real-life military servicemen.^{[citation needed]}