Ako Seto is one of the two main survivors in the aftermath of the catastrophic train derailment that opens the story. She is a junior high school student who was traveling with her classmates when the disaster strikes. When Teru Aoki first discovers her in the wreckage, she is unconscious and wounded, and he provides her with initial medical care while they are trapped in the dark, blocked tunnel.

In the beginning, Ako is portrayed as being in a vulnerable state, dependent on Teru for her survival and safety. She is disoriented and frightened by the situation, appearing to need protection from the immediate dangers, including the unstable and threatening behavior of their other surviving classmate, Nobuo Takahashi. However, this initial impression of fragility is quickly subverted as the narrative progresses. Once she recovers her strength, Ako reveals herself to be a character of immense inner strength, courage, and resilience. She is described as combative and someone who becomes indispensable to the pair's continued survival. Her moral compass remains unwavering throughout the story's horrors, establishing her as the moral centerpiece of the narrative; she consistently chooses to act with humanity and goodness even when surrounded by cruelty and chaos.

Her primary motivation, shared with Teru, is the desperate hope of returning to Tokyo to find her family and discover if they have survived the unknown cataclysm that has reshaped the world. This goal drives her forward through a landscape of ash,废墟, and hostile survivors. As she and Teru journey together, her role evolves dramatically from a rescued victim to an active and capable protector. When Teru falls seriously ill, the dynamic between them completely reverses, and Ako becomes the one who must take charge, find medicine, and fight for both of their lives. In a pivotal moment, she is forced to kill several townspeople in self-defense while securing medication for Teru, demonstrating her ability to commit violent acts when absolutely necessary, without succumbing to the senseless brutality that consumes many others.

Ako's most significant relationship is with Teru. Their bond is forged in shared trauma and mutual reliance, developing into a deep, subtle connection that transcends a simple romance and is rooted in a profound understanding of what they have endured together. Another key relationship, though a deeply negative one, is with Nobuo. He becomes deranged and attempts to assault her, an act that represents the utter breakdown of societal norms and morality in the face of fear. Her ability to escape him and later continue to function without being broken by the trauma highlights her formidable psychological strength. She also interacts with other survivors, such as the unstable Captain Nimura and the enigmatic boy Kikuchi, but her loyalty and commitment remain focused on Teru.

Throughout her development, Ako confronts the central theme of the story: the nature of fear. While many other characters surrender to their fear, losing their sanity or humanity, Ako feels fear but refuses to let it dictate her actions. She does not become numb or self-destructive like some survivors, nor does she turn predatory. Instead, she clings to hope and her own moral code, making her one of the few truly heroic figures in the desolate world. Her notable abilities are not physical superpowers but rather her psychological resilience, her quick thinking in crises, and her willingness to learn and adapt. When the situation demands it, she learns to handle a firearm and uses it effectively to protect herself and others, proving that her courage is backed by decisive action.