Mana Oshimizu, the younger sister of Nako Oshimizu, belongs to a large family including an older brother named Tomoya. She displays typical childhood behaviors, sometimes refusing to eat her vegetables. Nako cleverly manages these moments, inventing persuasive stories like the tale of an old man from the third district who would steal her bellybutton if she didn't comply. Nako also reads fairy tales to Mana at bedtime, reinforcing her caregiving role.

During the movie "Home Sweet Home," while her parents were away for work and Nako was responsible for the household, Mana ran away following an argument with Tomoya. Her disappearance triggered a search by Nako and Ohana Matsumae, coinciding with a power outage at Kissuisō inn where Nako worked. Frustrated by her parents' absence, Nako contacted them, urging them to prioritize family. Mana was eventually discovered on the roof of a department store by Nako and Ohana. They took her to Kissuisō to freshen up before returning home.

This incident became a catalyst for reconciliation. Mana's actions prompted her parents to return home early, allowing them to fulfill their original promise to accompany her on a school field trip. Her storyline illustrates the family's challenges and responsibilities, particularly Nako's crucial role in maintaining stability during parental absence. Mana's character deepens the exploration of Nako's home life and the broader themes of family dynamics.