Mai Niijima is a central character in her early twenties who has struggled to find her footing in life following the death of her mother, Takae Niijima. As the only daughter of the Niijima family, she grew up in a loving home but faced a profound crisis after her mother's unexpected passing. This loss, which occurred roughly ten years before the main events of the story, caused her to lose her motivation and direction, leading her to become a NEET, unemployed and without a clear path forward after graduating from a technical high school.

Mai is naturally depicted as shy, quiet, and reserved, often feeling anxious when faced with life's changes or challenges. She struggles with self-confidence and has difficulty connecting with others and building a career as a result of growing up with an absent mother and a father consumed by his own grief. However, at her core, she is a young woman capable of growth and resilience when given the right support and encouragement.

The primary catalyst for her change is the miraculous reunion with her mother, who returns as the reincarnated elementary school student Marika Shiraishi. Takae, now in Marika's body, provides Mai with the tough love and encouragement she desperately needs, helping her regain her confidence and find the courage to move forward. With this renewed hope, Mai successfully finds a job at a website creation company, marking a significant turning point where she begins to build her adult life.

Her role in the story is intrinsically linked to the theme of healing and learning to let go. While she shares in the joy of having her mother back in any form, a significant part of her arc involves confronting her deep-seated grief and the stunted development caused by losing her mother at a formative age. She must learn to build a life independent of her mother's presence, ultimately accepting that Takae's soul must move on for everyone's sake, including her own.

The most crucial relationship in Mai's life is with her mother, Takae. Even after Takae's death and subsequent reincarnation as Marika, her mother's love and support remain the most influential force in Mai's life, helping her to become a more positive and confident person. Her relationship with her father, Keisuke Niijima, is strained by years of mutual grief and emotional distance, as both were unable to properly communicate or support each other after Takae's death. The return of Takae helps to bridge this gap, forcing them to acknowledge their pain and begin to reconnect as a family.

Another defining relationship is with Renji Aikawa, a representative from a client company whom she meets through her new job. What begins as a professional interaction gradually develops into a deep romantic bond. This relationship is a cornerstone of her future, as she ultimately decides to marry him, signifying her ability to build new, meaningful connections and a life of her own separate from her past. Her development throughout the story is a journey from a state of loss and paralysis to one of strength and forward motion. She moves from being a grieving, unemployed young woman living in the shadow of her past to a confident employee, a loving partner, and eventually a wife and mother herself. The final stage of her growth involves the painful but necessary act of saying goodbye to her mother a second time, accepting that moving forward is the truest way to honor Takae's memory and sacrifice.