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In the second chapter of Danganronpa 2: Goodbye Despair, it’s revealed through the Twilight Syndrome Murder Case that Fuyuhiko had a sister who was murdered during their time at Hope’s Peak Academy. Though her name is not mentioned in the game, it’s revealed to be Natsumi when she's introduced in Danganronpa 3: The End of Hope's Peak High School.
Fuyuhiko and Natsumi don't interact on-screen in Danganronpa 3, but their care is evident for each other in their actions. Natsumi enrolled in Hope's Peak Academy as a member of the Reserve Course so she could stay near her brother. To Natsumi, being enrolled in the same school wasn't enough. In order to stay by Fuyuhiko's side, she attempted to become a Main Course student under the title of "Ultimate Little Sister." Fuyuhiko also refers to her by that title in one of his Free Time Events. Despite being unacknowledged by the academy, Fuyuhiko spoke highly of her and truly believed she earned her title. Prior to their enrollment, Natsumi was offered to take Fuyuhiko's role as heir to the Kuzuryu Clan. She declined, reasoning what made her amazing is being Fuyuhiko's sister. Fuyuhiko disagreed and found her a more worthy heir than him, and additionally deserving of the Ultimate Yakuza title. It is later revealed in Danganronpa 3 that she had a secondary motive of not wanting to hurt her brother's feelings by accepting his place as leader.
Fuyuhiko speaks fondly of his sister, despite being unafraid to state her flaws. He calls her selfish and arrogant and mentioned that she would constantly make him look like a fool. Despite this, he found her endearing and cared deeply for her. He held her in a very positive light, considering her to be better than him at everything. His concern for his sister is displayed in the aftermath of Natsumi's murder. In order to avenge Natsumi, Fuyuhiko murders the girl that took her life.
The two were raised in a turbulent household. Besides the criminal activity, their parents constantly fought and sometimes nearly killed Fuyuhiko in the aftermath of their altercations. It is not mentioned how Natsumi was treated by her parents, but it is noted that she was favored by many in the Kuzuryu Clan. Their unfair treatment does not seem to have caused any resentment toward each other, but both have similar attitudes that are likely the result of their upbringing.
Both Natsumi and Fuyuhiko put the other on a pedestal while being deeply entrenched in their own insecurity. They both struggle with anxieties of being worthy of each other and the Kuzuryu Clan. They also struggle with socialization, presenting a lone-wolf attitude despite their desire to make friends with people. It seems unconsciously they both push others away with insults, threats, and condescension.
Fanon has a range of portrayals of Natsumi and Fuyuhiko’s relationship. Many choose to go further in-depth while remaining canon-compliant, while a great deal also explores the possibility of Natsumi surviving. In the content where she survives, the Kuzuryu siblings are often shown growing past their insecurities and viewing each other on equal footing. Some Partners in Crime content is also very silly to explore more playful and carefree dynamics Natsumi and Fuyuhiko were unable to experience within the Kuzuryu Clan.
- Natsumi's relationship with Fuyuhiko at Danganronpa Wiki
- Fuyuhiko's relationship with Natsumi at Danganronpa Wiki
- Both Natsumi and Fuyuhiko have "milk" listed under their dislikes on their character profile, though Natsumi is not lactose intolerant like her brother.
- Natsumi is taller than Fuyuhiko, and he believes it's because she isn't lactose intolerant.
- Natsumi's character profile lists "her brother" as her likes.
- The first two syllables in Natsumi's name make the Japanese word for "summer"(夏 natsu), corresponding to how the first two syllables of her brother's name are the Japanese word for "winter"(冬 fuyu).
- Kuzuryu Clan refers to the ship between the two and Peko Pekoyama
- Natsumi Protection Squad refers to the ship between Natsumi, Fuyuhiko, Hajime and Peko