Iori Nagase is a central figure in her friend group, known as a bright and popular girl who serves as the president of the Student Cultural Society. She has a playful and energetic demeanor, often making perverted jokes and tongue twisters to keep the atmosphere light and fun, and she is considered one of the most beautiful girls in her year. Despite this cheerful exterior, Iori harbors a deep-seated psychological struggle rooted in a difficult childhood. She grew up with a single mother who had five different father figures, including a violent and abusive stepfather. To survive these turbulent environments and appease the expectations of those around her, Iori developed a habit of dramatically altering her personality to fit any given situation, becoming courteous with adults and cheerful with friends. Over time, this coping mechanism caused her to lose touch with any sense of an original or true self, leaving her unsure of her own identity.

This internal conflict is the primary driver of her character arc. Her role in the Cultural Research Club is not that of a traditional leader giving orders, but rather as a mood-setter who tries to keep everyone excited and prevent negative feelings. However, as the group is subjected to various supernatural phenomena that force them to confront their hidden sides, Iori’s carefully constructed facade begins to crumble. She struggles intensely with the belief that people only like the cheerful persona she projects and would reject her if they saw her real self. This culminates in her questioning her own feelings for Taichi Yaegashi, a close male friend whom she admires for his unwavering honesty and ability to stay true to himself. After they confess to each other, she ultimately decides to remain friends, partly due to the interference of the phenomena and her own doubts about whether he loves the ideal version of her. Later, she encourages Inaba Himeko, a fellow club member and her close friend, to act on her own feelings for Taichi, and the two maintain a strong, mutually respectful friendship built on a promise not to let their rivalry harm their bond.

Through the trials triggered by the entity Heartseed, Iori’s relationships with all club members deepen. Her development is marked by a gradual movement away from desperately searching for a lost "original self" and toward a more integrated and resilient identity. She eventually learns to embrace both the good and bad parts of her personality, allowing herself to express even negative emotions freely instead of suppressing them to please others. This growth is further solidified in later stages as she decides on a future career, aiming to become a teacher because she enjoys working with children, and begins studying education at university. Despite her chaotic emotional life, Iori possesses notable abilities in social navigation, being a surprisingly effective peacekeeper and source of wise advice when the group faces strife, though she also has comedic weaknesses such as not knowing how to slap someone properly, defaulting to an open-palm strike instead.