Nagisa Saitō is a high school student who works part-time at the Lemon Beach House, a small beachside restaurant run by the Aizawa siblings. She originally took the job because the location allowed her to pursue her passion for surfing, but her experience there is defined by her reaction to Squid Girl. Unlike every other character in the series, Nagisa genuinely believes that Squid Girl is a dangerous invader intent on taking over humanity, and she is consistently terrified of her. This fear sets her apart from the rest of the cast, who treat Squid Girl as an amusing and harmless presence.

Nagisa's personality is gentle, polite, and earnest. She is a kind and well-meaning person who tries to do the right thing, but her timidity and anxiety often overwhelm her. Her fear of Squid Girl leads her to overestimate the invader's plans and powers, and she is genuinely alarmed that the Aizawa sisters and others fail to take the threat seriously. Her primary motivation throughout the story is to protect humanity from what she sees as a real menace, and she feels a sense of responsibility to warn everyone around her. This creates a humorous dynamic where she becomes more invested in the idea of an invasion than Squid Girl herself often is.

In terms of her role in the story, Nagisa serves as the lone voice of caution and the straight man in a comedy that otherwise treats Squid Girl's antics as harmless. Her fear validates Squid Girl's ego, and Squid Girl enjoys having someone who is genuinely afraid of her, often exaggerating her abilities to keep Nagisa frightened. Nagisa's key relationships include her constant, one-sided conflict with Squid Girl, her confusion and exasperation with Eiko and Chizuru Aizawa for not sharing her concern, and her friendship with the other employees. Over time, her perspective shifts slightly: while she initially sees Squid Girl as the ultimate threat, she later comes to believe that Chizuru is actually the more dangerous person, a suspicion that is more accurate than her original fear. This change reflects a small but notable development in her understanding of the world around her.

Nagisa does not possess any special abilities or combat skills. Her notable traits are her unwavering vigilance, her capacity for genuine fear, and her love of surfing, which remains a secondary interest. She is not a fighter and relies on her persistent warnings and cautious behavior as her only means of opposing what she perceives as an invasion. Her character provides a consistent source of comic tension and serves as a foil to the more relaxed attitudes of the main cast.