Soyo Tokugawa, also known as Princess Soyo, serves as the younger sister of Tokugawa Shigeshige, the 14th shogun of the Edo shogunate. Rotten Maizou acted as her guardian after their father's death, becoming a fatherly figure to her. She owns a pet monkey; its offspring, Jugem Jugem, was gifted to Kyuubei Yagyuu. Initially embodying a polite, graceful, and well-mannered princess due to her sheltered life in Edo Castle, Soyo's personality unfolds to reveal deep curiosity, innocence, and naivety about the wider world. Befriending Kagura during an escape prompted by boredom and loneliness leads her to adopt more outspoken and subtly sadistic traits, influenced by Kagura's love for sukonbu and blunt humor. Shinpachi Shimura later identifies her alongside Kagura and Imai Nobume as one of the "three sadistic sisters" because of her occasionally sharp, often unintentional remarks. She remains cheerful and deeply caring within her close relationships. Her appearance features long black hair styled with straight bangs, soft features, and a height of 156 cm. She typically wears elegant kimonos and hair ornaments formally, but dons simpler attire like a green kimono with orange patterns when disguised as a civilian. Key relationships drive her development. She shares a close bond with her brother Shigeshige, affectionately calling him "Dear Brother" and frequently preparing tea for him, unaware of its foul taste. Her defining friendship with Kagura begins in a town park and leads to shared activities like sleepovers and mutual influence, including Soyo introducing Kagura to an Amanto suitor. She trusts Imai Nobume, her assigned guard during the Courtesan of a Nation Arc, and Okita Sougo, who protects her in the Shogun Assassination Arc. Her dynamic with Maizou mixes familial reliance with lighthearted complaints about his "sour" smell. Her story spans multiple arcs. In the *Courtesan of a Nation Arc*, she aids the Yorozuya and allies by facilitating their entry into Edo Castle and later distracts enemies to assist their escape from imprisonment. Her klutziness surfaces when she knocks herself unconscious trying to help Maizou. The *Shogun Assassination Arc* marks a turning point: Soyo nearly poisons Shigeshige unintentionally, then flees with him and Maizou via airship. Attacked by Kamui and the Harusame, Okita Sougo rescues her, but the trauma of witnessing the ship's destruction leaves her unable to tolerate animal innards afterward. Shigeshige dies on her lap after being poisoned, plunging her into prolonged grief. In the *Silver Soul Arc*, Soyo emerges from mourning to actively defend Edo. She evacuates samurai from Edo Castle and guards it alone against the Altana Liberation Army, narrowly surviving due to intervention by Katakuriko Matsudaira and the Shinsengumi. During the conflict, she confronts Nobunobu Hitosubashi over his role in her brother's death, demanding he return alive to report their success to Shigeshige's grave. After the war, Soyo ascends to political leadership as the second Prime Minister of Japan. She focuses on sweeping away oppressive systems, particularly those targeting women, and advocates for gender equality. Her governance includes supporting allies like Katsura Kotarou and coordinating security forces during the final battle against the Naraku and Tendoushuu remnants. She maintains her friendship with Kagura, sharing casual moments like eating dango with Maizou.