Hiyoko Saionji held the title of Ultimate Traditional Dancer, an honor inherited as heiress to the prestigious Saionji Clan. Her childhood involved intense training in Japanese dance under her grandmother's strict supervision, resulting in social isolation and severe bullying at Golden Light Girl's Institute. Classmates subjected her to life-threatening pranks, including placing pins in her footwear, scattering dead rodents in her bed, and poisoning her food. This fostered deep distrust and a defensive, cruel personality. She maintained affection for her father, who provided emotional support before her grandmother separated them to preserve family traditions. Enrolling in Hope's Peak Academy's Class 77-B, Hiyoko initially displayed mean-spirited behavior, particularly targeting vulnerable peers like Mikan Tsumiki. Her inability to perform basic tasks like tying her kimono highlighted her sheltered upbringing. A shift occurred when classmate Mahiru Koizumi assisted her, forming a sisterly bond. Hiyoko grew deeply attached to Mahiru, viewing her as "normal" among eccentric classmates, marking her first steps toward emotional vulnerability. During school events, Hiyoko witnessed pivotal tragedies. She discovered the body of Fuyuhiko Kuzuryu's sister in the Twilight Syndrome Murder Case alongside Mahiru, Ibuki Mioda, Mikan, and Sato. Sato's subsequent murder by Fuyuhiko devastated Mahiru, further bonding Hiyoko to her. These traumas foreshadowed her susceptibility to despair. Junko Enoshima's manipulation culminated in Class 77-B being lured into a trap. Forced to watch Chiaki Nanami's brutal execution via brainwashing footage, Hiyoko and her classmates descended into despair-induced insanity, laughing maniacally as their personalities twisted. As a Remnant of Despair, Hiyoko participated in global atrocities, including morale-boosting concerts with Ibuki to perpetuate The Tragedy. She and other Remnants later grafted parts of Junko's corpse onto themselves, symbolizing their devotion. Captured by the Future Foundation, Hiyoko entered the Neo World Program for rehabilitation. Memory-wiped and reverted to a virtual 16-year-old self, she participated in the Killing School Trip on Jabberwock Island. Her cruel behavior resurfaced—crushing ants, insulting peers, manipulating others—though her dependence on Mahiru remained. After Mahiru's murder in the game's second chapter, Hiyoko created a makeshift memorial, revealing genuine grief. Her development toward self-improvement began but halted in Chapter 3 when she witnessed Mikan Tsumiki kill Ibuki Mioda. Fleeing to a music venue, Hiyoko encountered Mikan, who slit her throat and tied her kimono incorrectly—a symbolic gesture implying disgrace. Post-Neo World Program, Hiyoko awoke from a coma. In the Hope Arc, she joined other reformed Remnants to intercept Ryota Mitarai's global brainwashing broadcast, protecting the Future Foundation. Though she panicked during combat, her survival was confirmed. Accepting exile for their past crimes, Hiyoko resettled peacefully on Jabberwock Island with her class.