Walker is a member of the US Army's Delta Force and leads his three-man squad through a devastated Dubai to find out what happened to Colonel John Konrad. Along with the 33rd Infantry, Konrad defected from the US Army and instead of following orders to leave Dubai, he stayed to help with the evacuation. But his attempts ended in complete failure. Six months after these events and two weeks after a mysterious looped radio signal reached the US military, Walker and his team are sent in for reconnaissance.
Although Walker is originally tasked with only a recon mission, he slowly turns it into his own quest of evacuating the remaining civilians and becoming a hero. But due to his own blindness and misguided intentions, the good he wants to do ends in even worse situations.
While there are several situations in Spec Ops: The Line where the player can choose what to do, it's often just a decision between what's bad and what's worse. Even the lesser evil can't stop Captain Walker from descending into madness.
A large part of Martin Walker's personality traits remain ambiguous or unknown to the player. Not much is known about his past other than that he has served in Kabul alongside Konrad, who saved his life during a mission. Therefore, Walker has strong feelings of trust and respect for him, to a point where he idolizes him and wants to be the hero he sees in Konrad.
At one point in the game, when one of his team mates questions Konrad's good intentions and asks how Walker can be so sure about him, he simply answers "Because I know the man", underlining his absolute confidence in Konrads nature.
During the events of the game, Walker witnesses and causes acts of cruelty and disturbing nature, which slowly change his attitude and behaviour. He gradually becomes edgier and more prone to violence, degrading not only physically but also mentally.
While Walker blames everyone but himself for his actions, it becomes clear he does suffer from self-loathing once he starts to hallucinate Konrad talking to him through a broken radio. Konrads voice is Walkers subconciousness trying to make him aware of his mistakes, but he keeps denying his fault.
Martin Walker is voiced by Nolan North.
His name could be a homage to Martin Sheen, who played a similar character in Apocalypse Now, another interpretation of the Heart of Darkness novel.
According to Spec Ops: The Line lead writer Walt Williams, Walker may have died during the helicopter crash in the beginning of the game, meaning the events the player goes through are actually Walker's personal hell, though the game does allow multiple interpretations of the story.
During Walker's hallucinations, the screen will fade to white instead of black, which casts a shadow of uncertainty over one of the endings which fades out in white.
[about Konrad] "But the facts don't lie. The man's a fucking hero."
"I thought we were rescuing people..."
"I wanna know what the hell is going on in this city."
"Our mission went from recon to rescue the minute we found US soldiers butchered."
[to Sgt. Lugo] "Orders ain't worth following if it means leaving people to die."
"Konrad's still alive, I think he is. I know he'd agree with me."
"I'm not about to shoot a US soldier."
"It's clear the 33rd is no longer acting as part of the US army."
[about the Radioman] "I actually think I met him once, in Kabul. He was a reporter traveling with the 33rd. Weird guy. The kind you didn't want loose in a firefight."
[when asked to identify himself by an unknown voice] "Captain go fuck yourself."
"I think we landed in the middle of a fucking warzone."
"We have no choice. You've seen what has become of the 33rd."
"This isn't just about finding Konrad anymore. It's about doing what's right."
[to a dying soldier] "You brought this on yourself."
"I'm gonna make these bastards pay for what they've done."
[about Riggs] He fucked us, fucked everybody. Thanks to him everyone in Dubai will be dead in a few days."
"I can do whatever the fuck I want"
"You're not real. This is all in my head..."
"No everything, all this, this was your fault"
"I... I didn't mean to hurt anybody"