Keiji Kiriya is the protagonist of the original source material. He begins as a raw recruit in the Japanese branch of the United Defense Force, a global military organization formed to combat an alien invasion. Having voluntarily enlisted, he is inexperienced and unprepared for the harsh realities of frontline combat, where soldiers are equipped with powered exoskeleton suits known as Jackets.

Initially, Kiriya is a reluctant and somewhat cynical soldier, aware of his own lack of skill and the terrifying nature of the enemy, an alien race known as the Mimics. He does not possess the mindset of a natural-born hero. However, his personality undergoes a profound transformation after his first battle, in which he is killed. Instead of dying, he awakens the previous day, trapped in a time loop that forces him to relive the same day and the same battle over and over again. This experience, while psychologically exhausting, becomes the crucible for his development. He evolves from a frightened amateur into a hardened and highly capable warrior.

Kiriya's core motivation shifts from simple survival to a desperate need to break free from the endless cycle of death and rebirth. As he repeats the same day hundreds of times, his goal becomes more focused on understanding the Mimics and finding a way to achieve a permanent victory, thereby ending the war and the loop. His journey is not driven by glory or patriotism, but by a deep-seated desire to reclaim his own life and prevent further loss.

His most significant relationship is with Rita Vrataski, a legendary soldier known as the Full Metal Bitch who has also experienced a similar time loop in the past. She becomes his mentor, training him to use the loop to its full potential and teaching him the combat skills necessary to survive. Their bond evolves from that of a student and a distant, almost mythic instructor to a deep partnership and camaraderie forged by a shared and isolating experience that no other soldier can comprehend. The supporting soldiers in his unit serve as recurring touchstones in each loop, but the narrative focuses primarily on Kiriya's solitary struggle and his connection with Rita.

The primary ability Kiriya develops is not a superpower, but a consequence of the time loop itself. He retains the memories, knowledge, and muscle memory from every previous cycle. This allows him to learn from his countless deaths, gradually mastering combat techniques, predicting enemy movements, and refining his strategic approach to the same battle. He becomes a hyper-competent fighter, not through innate talent, but through an exhausting accumulation of experience over thousands of repetitions. He learns to use his Jacket armor with expert proficiency and becomes adept at identifying and eliminating key Mimic targets. The psychological toll of this process is a notable part of his character, as he endures the trauma of dying repeatedly and the emotional burden of watching his comrades fall, only to see them alive again at the start of the next loop.