Natsumi was the younger sister of the Kuzuryu Clan's heir, Fuyuhiko, though she was considered by many to be more worthy of the role. The Clan even believed she was the reincarnation of her deceased uncle who was known as the strongest Kuzuryu in history. She was given the chance to take her brother's place, but she declined due to caring about her brother. She didn't wish to lead the Clan, believing she was so amazing because she was Fuyuhiko's sister.
Three years before attending Hope's Peak Academy, Yasuhiro Hagakure extorted money from a wealthy female client, but as a result he became a target of a mafia and had a debt of over 8 million. In Danganronpa: Trigger Happy Havoc IF, it's heavily implied that the female client was Natsumi, mentioned as "that Kuzuryu girl". Yasuhiro had apparently abortively attempted to sell Makoto Naegi's organs to the clan.
After her brother attended Hope's Peak Academy, Natsumi felt she was left behind and began to call herself "Ultimate Little Sister", believing she was worthy of the title due to her brother being the Ultimate Yakuza.
