Masuzu Natsukawa is a main female protagonist in the series Oreshura. She is a first-year high school student who has recently returned to Japan after spending nine years living overseas, and she is the older sister of Mana Natsukawa. She is widely regarded as the school beauty, with long gray hair and blue eyes, and she is constantly the recipient of romantic confessions from male students. To those who do not know her well, Masuzu presents a quiet, reserved, and cold demeanor, often staring out the classroom window and avoiding conversation. She is brutally honest about her beliefs, sometimes to the point of seeming uncaring.

In private, however, Masuzu reveals a very different personality. She is devious, manipulative, bold, and acts more mischievously when not in the public eye. Her true character is sadistic in nature, and she admits that she enjoys seeing the depressed or uncomfortable expressions of others. Despite this, she maintains her pleasant facade because she fears that showing her real self would cause people to hate her. Like the protagonist Eita Kido, Masuzu genuinely despises romance and considers love to be troublesome. She sees the constant romantic attention she receives as a nuisance, not a compliment.

Motivated by her desire to stop the endless confessions, Masuzu approaches Eita, the only boy at school who seems completely indifferent to her beauty, and proposes that they become a fake couple. When Eita refuses, she blackmails him with his embarrassing secret notebook to force him into the arrangement. This false relationship becomes the central engine of the story, as the news spreads throughout the school and provokes a rivalry with Eita's childhood friend Chiwa Harusaki, who truly loves him.

Masuzu's family background is complicated and deeply influences her actions. She comes from a wealthy family that lives in Sweden, but she does not have much money of her own. She has a strained relationship with her father, who remains in Sweden, and she refuses to return there because she is searching for her mother, who was ostracized by him. This difficult home life and her emotional distance from her family have shaped her cynical worldview. She also does not get along well with her half-sister Mana.

Over the course of the story, Masuzu experiences significant development. In the fourth volume of the original novels, she reveals that she initially hated Eita because he was the first person who was not charmed by her appearance and did not become paralyzed simply by looking at her. Her initial coldness toward him gradually gives way to genuine, complicated feelings. As she becomes more involved with Eita, Chiwa, and the other characters, she begins to open up and show more of her vulnerable side. Her journey is one of self-discovery and learning to let go of the past. In the ending of the anime, Eita ultimately chooses her over the other girls.

Masuzu is notably a terrible cook, a trait she shares with Chiwa, and she loves manga, listing JoJo's Bizarre Adventure as one of her interests. She does not possess any supernatural abilities, but she relies on her considerable charisma, sharp intellect, and manipulative social skills to navigate the complicated situations she finds herself in. She uses her understanding of human nature and her willingness to blackmail to maintain control over her fake relationship with Eita. Throughout the series, she also tries to balance her budding genuine feelings, her personal ambitions, and the rivalries and friendships that develop around her.