Sanae Nagatsuki attends high school as Eiko Aizawa's childhood friend, often seated behind her in class. She lives alone on a large property, with her parents' absence unexplained, and owns a pet dog named Alex who grows jealous of Ika Musume. Sanae develops an intense obsession with Ika Musume, initially expressed through constant stalking, photographing, and attempts to dress her in cosplay outfits; this later shifts to collecting Ika-themed memorabilia. She harbors intense, possessive feelings towards Ika Musume, frequently imagining romantic scenarios and reacting with jealousy to others interacting with Ika. Sanae meticulously learns Ika's preferences, discovering her favorite food is shrimp, which motivates Sanae to cosplay as a shrimp twice to get closer. Despite Ika's consistent rejections—often involving tentacle attacks or ink-spraying—Sanae interprets these reactions as attention and finds enjoyment in them. Through repeated exposure to Ika's tentacle attacks, Sanae gradually develops enhanced physical capabilities, including greater endurance, agility, and the ability to dodge effectively. She demonstrates the ability to swim long distances using the butterfly stroke. Harsh training under Chizuru Aizawa further augments her strength, enabling her to chop through 15 tiles. Sanae's character evolves in managing her obsession. Initially prone to physical advances, she eventually stops attempting direct contact and recognizes that Ika's other friends, like Kiyomi Sakura, are not rivals. She transitions to admiring Ika from a distance while maintaining dedication. In serious situations, such as helping Eiko retrieve a doll, Sanae remains calm and cooperative around Ika. Her hobbies include writing, and she maintains a wardrobe originally filled with cosplay outfits. Sanae is the most recurring non-titular character in the series, appearing in 36% of the manga chapters.