Masumi Ageo is a relatively new teacher at Futaba Kindergarten, where she is responsible for the Tulip class. She is twenty three years old and typically appears with medium length brown hair and glasses. Her physical demeanor is meek and anxious, and she is prone to experiencing panic attacks when confronted with large groups of children, a regular occurrence in her profession as a kindergarten teacher.

Her personality is defined by extreme nervousness and timidity. However, she possesses a distinct personality shift that occurs when her glasses are removed and her vision becomes blurred. In this state, Ageo becomes highly aggressive, fearless, and outspoken, a stark contrast to her usual self. This dual personality trait is a central aspect of her character and is often triggered unintentionally by the antics of the protagonist, Shinnosuke Nohara. Her motivation as a teacher is to perform her duties competently despite her anxieties, and she gradually works toward becoming more comfortable in her role. Over the course of the series, she becomes somewhat less introverted, though she never fully sheds her core shyness.

In the story, Ageo serves as a supporting faculty member at the kindergarten. Her interactions with the children, especially Shin-chan, frequently lead to comedic situations, often involving her glasses being removed and her aggressive alter ego taking over. She also has a notable fixation on her colleague, Kuroiso, with the nature of that fixation ranging from admiration to romantic interest depending on the adaptation. Her relationships with the other teachers, such as Ume Matsuzaka, and the kindergarten director are professional but occasionally strained by her anxious behavior.

Her development is gradual and tied to her growing confidence as a teacher and her ability to handle her own quirks. While she remains inherently nervous, her experiences with the children and staff help her become slightly more extroverted and capable over time. Her most notable ability is the drastic personality transformation that occurs without her glasses. This change allows her to perform actions and speak with a forcefulness that her normal self cannot muster, though the effect is temporary and reverses when her vision is restored. She also appears in several of the series animated films as a supporting character, maintaining the same personality traits.