Mayumi Katō, a sixth-grade student in Kamohashi Elementary’s class 6-3, anchors the Hopeless Squad as its tallest member, her brown hair styled in spiky bangs and a ponytail fastened by a hair bobble. Clad in a t-shirt and mini skirt, her perpetually concerned expression mirrors her role as the trio’s cautious counterbalance to Airi Ogata’s reckless spontaneity and Shiori Itou’s cunning stratagems. Though equally smitten with classmate Shinya Satō, Mayumi tempers her friends’ extremes—quietly disapproving of Airi’s underwear-free antics or Shiori’s sly eraser thefts—yet hesitates to confront them, prioritizing group harmony over personal objections.

Her affection for Satō traces to early childhood, predating her elementary friendship with Airi, marking her as the first in the trio to harbor feelings for him. This history fuels her quiet conflict: torn between loyalty to her friends and a yearning for Satō’s attention, she navigates both with equal fervor. During the squad’s chaotic schemes—spreading rumors about Satō’s preferences or scrambling over a mistaken photo—Mayumi often proposes pragmatic fixes, only to watch them unravel under her friends’ exuberance.

In group crises, she physically intervenes, shielding Airi from wardrobe mishaps or subtly countering Shiori’s ploys, her situational awareness clashing with childlike naivety. While she notices Shiori’s underhanded tricks, she opts for silent vigilance, avoiding direct clashes to preserve their bond. Her presence weaves restraint into the squad’s antics, embodying reluctant moderation amid relentless chaos, her expressions oscillating between shock, resignation, and fleeting determination.