Nagisa Shiozaki teaches kindergarten in Tokyo. Her past holds a significant romantic disappointment: her first love, a tennis coach from her student days, left her for another woman. This experience caused deep-seated trust issues and a reluctance to pursue new relationships despite her desire for companionship. She harbors skepticism toward men due to this heartbreak and encounters with persistent, disrespectful suitors, including a muscular physical education teacher and a wealthy real estate agent who ignored her boundaries.

Her personality blends wariness with underlying vulnerability. She displays tsundere traits, often pushing away Masuo Fugano, a salaryman genuinely interested in her, through harsh words or dismissive actions even as her feelings for him grow. This behavior stems from her fear of repeated emotional pain. Nagisa frequently misinterprets Masuo's actions due to his chronic bad luck and tendency to land in compromising situations, especially those involving his coworker Miho Hamaoka, who harbors romantic feelings for him. These misunderstandings create recurring obstacles.

Her development involves gradually overcoming emotional barriers. Initially, she rejects Masuo's advances outright and doubts his sincerity. Over time, she acknowledges her feelings but struggles to express them, leading to poor communication: she dismisses his explanations or discovers partial truths reinforcing her suspicions. Key moments include discovering Masuo's unintentional first kiss with Miho and a significant misunderstanding when Masuo shelters a pregnant woman out of compassion. Nagisa initially receives misinformation suggesting the child is Masuo's but eventually determines the truth independently.

Her relationship with Masuo evolves across adaptations. In the manga and OVA, she faces romantic competition from other suitors, contributing to her hesitancy. The anime streamlines this, focusing on the love triangle with Miho and culminating in a train-station farewell where Nagisa and Masuo share their first kiss before his transfer, implying reconciliation. The manga continues beyond this, showing Nagisa ultimately committing to Masuo, resulting in their marriage and the birth of their daughter, Yuka.

Secondary relationships influence her journey. Matchmaker Namiko Isobe occasionally facilitates communication, most notably convincing Nagisa to hear Masuo out before his transfer. Nagisa's interactions with Miho are marked by jealousy and conflict, particularly when Miho's affection creates situations Nagisa misinterprets as betrayal. These relationships highlight Nagisa's insecurities and gradual emotional growth.