Nagisa Tennōji, commonly called Nagi, is one of the four main characters in the series. She is a second-year high school student and a classmate of Run Momoki and Yūko Nishi. Nagi has brown hair that she usually wears in sports-style pigtails and large round glasses, as she is quite nearsighted. When she takes off her glasses and lets her hair down, her appearance changes dramatically. She typically wears a beige cardigan over her school uniform.

Nagi is highly intelligent and observant, often making dry, ironic remarks about the foolishness or antics of her friends. She serves as the sensible, level-headed foil to the more flighty and impulsive members of her group. Despite her generally composed demeanor, she has some personal insecurities, particularly about her weight, and she occasionally tries to go on diets, though she dislikes exercise and rarely succeeds. She also dislikes extreme temperatures, especially cold weather.

Her primary motivation is to maintain a sense of normalcy and order amidst the chaos generated by her friends. She is a loyal friend who genuinely cares for Run, Yūko, and the younger first-year student Tōru Ichii, even when she is exasperated by their behavior. Nagi's role in the story is that of the straight man, providing witty commentary and keeping the group grounded. Her relationships are built on mutual affection and teasing; she shares a close friendship with Run and Yūko, and she often interacts with Tōru as well.

Over the course of the series, Nagi remains reliably pragmatic, though she occasionally reveals a playful side and participates in the group’s adventures. She does not undergo major dramatic changes but consistently acts as the calm, intelligent anchor among her friends. Her notable abilities include her sharp intellect, quick wit, and her talent for delivering cutting remarks that defuse or highlight the absurdity of situations.