Keisuke Niijima is the protagonist of the story, a man whose life was shattered by the sudden death of his wife, Takae, in a car accident. For the ten years following her passing, he existed in a state of profound grief and stagnation. He became a gloomy, despondent shell of his former self, feeling as though he was no longer truly alive. During this time, he shared a home with his daughter, Mai, but they lived largely separate lives, with Keisuke unable to build a proper relationship with her or move forward as a parent. He provided only financial support, and the emotional heart of the household was gone, causing time to seem as if it had stopped for the Niijima family.

Keisuke is defined by his deep, unwavering love for his late wife. This devotion forms the very foundation of his personality and motivations. He is, at his core, a devoted husband who never remarried or changed his feelings for Takae. This powerful emotional dependence on her is what initially incapacitated him. However, the story reveals that before the tragedy, he was a more cheery and perhaps somewhat rash individual, a side of himself he gradually rediscovers. When an elementary school girl named Marika Shiraishi appears on his doorstep claiming to be the reincarnation of his wife, Keisuke is initially dismissive but quickly becomes a believer when she proves her identity with intimate knowledge only Takae could possess. This miraculous event reignites his will to live. His joy at being reunited is so immense that it can lead to problematic behavior, such as forgetting the sensitive nature of his wife now inhabiting a child's body and acting in ways that could be easily misconstrued. His deep-seated motivation is to reclaim the happiness he lost and to make new memories with his family, even in this unconventional form.

Within the story, Keisuke serves as the emotional anchor for the family's healing process. His primary role is that of a grieving husband and father who must learn to live again. His key relationships are central to this journey. His bond with his daughter, Mai, is initially cold and distant, but the return of Takae acts as a catalyst for them to reconnect and rebuild their father-daughter relationship. His relationship with Konomi Moriya, a colleague at work, also becomes significant as she helps him develop a more positive outlook and a degree of self-dependence, even leading to a complex situation where they stage a fake proposal to help Takae move on. The most crucial relationship is, of course, with Takae, who exists in the body of Marika. He initially clings to this second chance, but his character development forces him to confront the fact that he must eventually let go of his grief for the sake of everyone, including the young girl whose body his wife inhabits.

Keisuke undergoes a profound transformation. At the beginning, he is a man unable to function, but he gradually regains his zest for life through his secret meetings with Takae, who makes him handmade bentos and gives him the emotional support he has been missing. A major turning point in his development is the painful decision to let go. After an emotional conversation with Takae, he realizes he must learn to live without her again and become the parent Mai truly deserves. He begins performing small acts of independence, like making his own lunches, and ultimately participates in a staged plan to convince Takae that he has moved on, allowing her soul to finally find peace. He has no supernatural or combat abilities. His notable traits are his profound capacity for love, his emotional vulnerability, and his determination to heal, even when it requires the most difficult sacrifice of letting go a second time.