Kinue Hayase is the mother of the protagonist, Masamune Makabe. Despite being forty-two years old, she possesses an exceptionally youthful appearance that often leads strangers to mistake her for her son's younger sister. She is notably short, has a small frame, light brown hair that curls at the ends, and brown eyes. Her typical attire includes an apron and a headband decorated with a dog's paw print, which contributes to her childlike look. Her name is also spelled as Hayase Kinue.

Her personality is defined by an overwhelmingly positive and doting nature, particularly towards her son. She is a devoted mother whose behavior often mirrors her youthful appearance, as she frequently displays a child-like demeanor. A prominent aspect of her character is her deep-seated need to express affection through food. She constantly prepares high-calorie, often fried, dishes in an attempt to make Masamune happy, although this frequently leads to him becoming annoyed. When her son rejects her food offerings, she experiences dramatic mood swings, transitioning instantly from happiness to profound sadness, and often concludes that Masamune hates her, a belief that is entirely untrue and driven by his desire to avoid gaining weight.

Kinue's primary role in the story is that of a loving but overbearing mother who is directly responsible for her son's difficult childhood. Her relentless feeding of calorie-rich meals was the main reason Masamune became overweight as a child, an experience that traumatized him and motivated his extreme physical transformation and subsequent quest for revenge. Even after he returns home as a fit and handsome high school student, she continues her old habits, forcing him to meticulously manage his diet and exercise to maintain his new physique. Her interactions with her son are characterized by this loving friction; she affectionately calls him "Maa-kun" while he teasingly refers to her as an "old loli" or "loli hag".

Within the story's relationships, Kinue's world centers on her family. She is the mother of two children: Masamune Makabe, whose surname is different because he took his grandfather's last name, and Chinatsu Hayase, her daughter who is often seen relaxing at home and enjoying her mother's cooking. Her interactions extend beyond the family as well. On one occasion, she spots Yoshino Koiwai and Neko Fujinomiya in her neighborhood. Recognizing their school uniforms as matching her son's, she forcefully invites them into her home for dinner, which leads to an awkward situation for Masamune, highlighting her inability to read social cues when her maternal instincts take over.

Kinue's character shows a notable development later in the story. Her cheerful and overbearing personality is tested when Masamune begins to rebel against her authority. After he completely stops listening to her and his sister and leaves the family home to pursue his own goals, Kinue becomes deeply depressed. In a stark contrast to her usual energetic self, she loses all motivation, hides under her blankets, and isolates herself in her son's room, staying in this state until he finally returns. This moment reveals a more fragile and vulnerable side beneath her perpetually cheerful exterior. Her notable abilities are not physical or combative, but rather culinary, specifically her talent for creating high-calorie, fattening dishes that have a powerful influence on the story's central conflict.