Hizume Arima is a high school student at Kaminoki High School and a central member of the cheerleading group formed by Kohane Hatoya. Her significant cheerleading experience began before high school with a community team during junior high. Recognized for exceptional skill and nicknamed the "Iron Base," her intense focus on technical perfection led to isolation from teammates unable to match her abilities. This resulted in persistent harassment that forced her to quit cheerleading just before entering high school, creating deep reluctance to rejoin any squad.

She is the tallest among the main characters, with long bluish-purple hair typically secured by a distinctive horseshoe-shaped hairpin. Her everyday demeanor often appears serious and expressionless, contrasting sharply with the vibrant smiles she displays during performances. Socially polite and intelligent, she struggles with interpreting figurative language, frequently taking statements literally. This literal-mindedness, combined with her reserved nature, initially made her seem unapproachable to classmates, though many secretly admired her.

Her development begins when Kohane persistently encourages her to return to cheerleading. Initially skeptical and haunted by past experiences, she gradually reengages through training Kohane and Uki Sawatari. She assumes a de facto leadership role, designing routines, assigning stunt positions, and overseeing physical conditioning. Despite her expertise, she nominates Kohane as club president, valuing Kohane’s innate ability to motivate others.

Key aspects of her growth include confronting social anxieties. She actively seeks advice on appearing friendlier and learns classmates avoided her out of intimidation, not dislike. Her deepest fear centers on abandonment, manifesting in distress when Kohane suggests quitting cheerleading. She reconciles with her past through encounters with former teammates, including the Nekoya twins, during a cheer tournament. These interactions reveal her old squad harbored no ill will and respected her growth, allowing her to move forward.

Family influence includes an older brother in college who inspired her initial interest in cheerleading. Personal quirks include a late realization about Santa Claus’s nonexistence and attempts at manga creation despite limited artistic skill. Her journey culminates in embracing the smaller team dynamic, where mutual support helps her overcome isolation and rediscover joy in cheerleading as a collaborative endeavor.