Mashiro Tsukinomori, a second-year high school student, is the daughter of Honeyplace Works president Makoto Tsukinomori. As Akiteru Ōboshi's cousin and childhood friend, their relationship grew distant over time, though her name meaning "pure white" reflects her core traits.

Typically shy and reserved around strangers, Mashiro directs coldness and sharp remarks exclusively at Akiteru due to an incident where he accidentally saw her underwear during a family reunion. This dynamic transforms after she confesses her feelings to him via LIME following his welcome party. She then actively pursues his affection, abandoning hostility for overtly girlfriend-like behavior. While declaring trust in his fidelity, she competes fiercely with rivals like Iroha Kohinata—instantly sending Akiteru a swimsuit photo upon learning Iroha did the same.

Secretly, Mashiro is the celebrated light novel author "Namako Makigai," whose UZA Bunko award-winning series sold over three million copies. Her writing career is distinguished by sarcastic wit, encyclopedic sea creature knowledge, innovative style, and strong character development, yet she consistently rejects manga or anime adaptations.

As scriptwriter for the 05th Floor Alliance game group—including Akiteru, Ozuma Kohinata, Iroha Kohinata, and Sumire Kageishi—she joins after Akiteru tricks her into attending their welcome party. Her fear of bullying leads her to confide in the members.

Mashiro's relationship with Iroha evolves from tension to mutual respect when Iroha defends her against past bullies during a mall trip. Though considering Iroha a friend, Mashiro remains vigilant about her romantic interest in Akiteru and actively rivals her. She visibly worries and saddens when Akiteru seriously contemplates marrying Sumire Kageishi for strategic advantages.

Her family includes father Makoto and mother Kaigetsu Tsukinomori. Family arrangements facilitated her reunion with Akiteru, catalyzing their complex relationship.