Major William Cage, often referred to as Bill, is the central character of the film. He holds the rank of Major within the United Defense Force, but his role is that of a public relations officer rather than a frontline soldier. Before the war against the alien Mimics, Cage was an advertising executive whose firm collapsed due to the conflict, leading him to use his skills in media and propaganda for the military. He played a key role in promoting the exo-suit technology and crafting the public image of war heroes to boost recruitment and morale.

At the outset, Cage is portrayed as a charismatic but ultimately cowardly and self-serving individual. He is an officer who has never seen combat and is deeply averse to the idea of facing it. When ordered to join the front lines to film the Normandy invasion, known as Operation Downfall, he attempts to use blackmail to avoid the assignment. This act of insubordination leads to his arrest, a demotion to the rank of private, and being forcibly sent into the battlefield as part of a unit called J-Squad.

His primary motivation shifts drastically over the course of the story. Initially, he is driven purely by self-preservation, desperate to escape the combat zone and return to his safe, comfortable life. This motivation is shattered when he is killed in his first battle, only to awaken at the start of the previous day, trapped in a time loop. From that point forward, his goal becomes finding a way to break the loop and win the war, first to save himself but gradually evolving into a genuine desire to protect his comrades and all of humanity. His role in the story is that of a reluctant soldier transformed into an unlikely hero. He becomes the key to defeating the Mimics, as his unique ability to reset the day gives him countless opportunities to learn, train, and strategize.

His relationship with Sergeant Rita Vrataski is the most significant driver of his development. Rita, a legendary war hero known as the Angel of Verdun who previously possessed the same time-looping power, becomes his mentor. Their relationship is initially tense, with Rita subjecting him to brutal and repeated training exercises, often killing him at the end of a loop to force him to start over and improve. Through this relentless process, she forges him from a clueless amateur into a skilled and capable warrior. Their dynamic evolves from teacher and student to a deep bond of mutual respect and trust, as they become the only two people who can truly understand the unique horror and burden of living through the same day hundreds of times.

Cage undergoes a profound character transformation. He begins as an arrogant, haughty, and fearful individual, a classic example of an over-ranked soldier with no real combat experience. Through the crucible of hundreds of deaths and repeated days, he develops immense courage, resilience, and strategic brilliance. He becomes numb to pain and the fear of death, replacing them with a focused determination. He learns to master the exo-suit, memorize enemy movements, and predict the actions of those around him with near-superhuman precision. By the end of his journey, he has shed his cowardice entirely, becoming a selfless and heroic figure willing to sacrifice himself to destroy the Mimic Omega and end the war.

His defining ability is the power to reset the day upon his death. He acquires this power after killing an Alpha Mimic, a rare and powerful creature, and being drenched in its blood. This blood integrates him into the Mimics' hive network, causing the Omega, the central alien brain, to reset time every time he is killed, pulling him back to the start of the previous day with all his memories intact. This power allows him to learn from every mistake and death, making him a superhumanly effective soldier. His abilities are not supernatural but are the result of countless loops of "taught by experience," where he hones his combat reflexes, memorizes battlefields, and masters the use of his powered armor to a degree no ordinary soldier could achieve.