Kei Asai is the main protagonist of the story, a high school boy living in the special town of Sakurada where most inhabitants possess unique supernatural abilities. He is a tall and slender teenager with bluish-gray hair and eyes. Kei lives alone, having made a conscious decision as a child to remain in Sakurada permanently, a choice that resulted in his family's memories of him being erased.

Kei possesses the ability called Memory Retention. This power grants him a perfect and unalterable memory, allowing him to retain and accurately recall any piece of information from any point in his life. The most critical aspect of this ability is that it makes him immune to external memory manipulation, including the effects of Misora Haruki's Reset ability, which rewinds time. While everyone else in the town, including Misora herself, loses their memories of the erased timeline, Kei alone remembers everything that occurred before the Reset. His keen observational skills and quick decision-making complement this power, as he is able to grasp complex situations rapidly and act without hesitation.

Kei's personality is defined by a calm, analytical, and distinctly stoic demeanor. He rarely displays overt emotion and often speaks in a measured, philosophical tone, which can make him appear detached. However, he is driven by a powerful, almost idealistic motivation: to erase the sadness of others. He expresses a deep-seated desire to wipe away the tears of every crying child and to help those in trouble. Interestingly, Kei himself does not view his own helpful actions as genuine kindness. He introspectively labels them as a form of selfishness, suggesting that his drive to help others is a personal compulsion that merely appears as kindness to the outside world. He is also known to give sarcastic answers and has a fondness for sweet food.

His role in the story is firmly established through his partnership with Misora Haruki, a girl whose ability to reset time by up to three days is rendered strategically invaluable when combined with Kei's perfect memory. Together, they form the core of the Service Club, a group sanctioned by the town's governing Bureau to use their abilities to solve problems and help other residents. Kei acts as the primary strategist and decision-maker, using his unique memory to analyze past timelines and devise optimal solutions that Misora can then execute with a Reset.

Kei's key relationships define his journey. His relationship with Misora Haruki is central to the narrative. He sees beyond her emotionless exterior and understands that her power has left her disconnected from her own identity. His initial goal is to help her become an ordinary girl capable of experiencing a full range of human emotions. Over time, their professional partnership deepens into romantic feelings, and they eventually become a couple. Another crucial relationship is with Sumire Souma, the class representative who first introduced him to Misora. Sumire, who has the ability to see the future, fell in love with Kei because she saw his future as the sweetest. Her eventual death, which occurs after a Reset, becomes a profound source of guilt and a primary motivation for Kei, who vows to find a way to revive her.

Throughout the series, Kei undergoes significant development, largely driven by the consequences of his seemingly perfect logic. Initially confident in his ability to engineer the best outcome, he is deeply shaken by Sumire's death, which he blames on his own decision to use Misora's power. This event forces him to confront the limits of his utilitarian approach and the heavy responsibility of manipulating time and the lives of others. He grapples with his own ideals of what it means to be a good person, constantly struggling with the ethical implications of his and Misora's actions. The story ultimately presents him not as an infallible god-character, but as a determined individual wrestling with complex moral questions and the unforeseen costs of his choices.