Masumi Ageo teaches at Futaba Kindergarten, initially substituting for the injured Ume Matsuzaka. After Matsuzaka's recovery, Ageo becomes the permanent instructor for the Tulip class (also known as the Cherry class). She is approximately 23-24 years old. Her personality undergoes a dramatic shift dictated by her eyewear. Wearing glasses, Ageo displays extreme shyness, social anxiety, and nervousness, especially around large groups of children. This timidity leads to difficulty maintaining classroom discipline, as students exploit her hesitation to extend playtime or ignore instructions. Removing her glasses—accidentally or deliberately—causes severe myopia and a complete transformation. Blurred vision accompanies an aggressive, confrontational, and outspoken persona, unafraid to voice criticisms or enforce order intensely. This version typically feels deep embarrassment upon regaining her glasses and reverting to her shy self. Ageo harbors romantic feelings for Kuroiso, Ai Suotome's chauffeur and bodyguard. The intensity of this fixation varies across adaptations, from admiration to overt romantic interest. Speculation about a potential marriage between them exists within the series, though unconfirmed. Her surname originates from Ageo City in Saitama Prefecture. She suffers from acrophobia, an intense fear of heights, and demonstrates proficient computer skills by maintaining a personal blog or website for journaling, reportedly lacking visitors. She expresses a fondness for viscous or slimy textures. Subtle development occurs throughout the series, with Ageo gradually displaying marginally increased confidence and social engagement while retaining her fundamentally introverted nature. She resists wearing contact lenses due to discomfort with touching her eyes and a belief they might cause mental decline. Physically, she has medium-length brown hair, fair skin, and dark eyes, though later depictions often obscure her eyes behind her glasses' reflective lenses. An unexplained continuity detail notes her near-identical resemblance to an unnamed Tulip class teacher appearing in an earlier episode before her formal introduction.