Sanae Shiratori is a war orphan whose parents perished during the Zaal Empire's invasion of Earth. She witnessed their deaths firsthand when Zaal forces destroyed her family home, forcibly restrained by civilians as she watched helplessly while the building collapsed. This trauma defined her early survival in the post-apocalyptic world. She eventually encountered other orphans, bonding deeply with the younger Ochame over shared loss; Sanae provided Ochame shelter and companionship. During this time, she met Kento Tate and his group when they shared stolen food. Sanae objected to the theft on principle but accepted it out of necessity for Ochame's survival. Later, discovering Danji Hiiragi injured, she brought him to Kento's hideout for treatment, marking the start of their group dynamic.

At Adalus Base, Sanae assumes a nurturing and supportive role. Her character embodies kindness, a strong moral compass, and resilience, serving as the group's emotional anchor. She maintains a determined cheerfulness despite their harsh circumstances, reflecting her commitment to hope. Sharing responsibilities with Kento, she cares for the younger orphans—Ochame, Jiro, and Manabu—acting as a maternal figure managing daily life, cleanliness, and emotional support. Her empathy frequently surfaces, such as objecting to unethical actions like stealing, though she compromises when necessary for the group's survival.

Relationships are central to Sanae. She develops a profoundly close bond with Ochame, treating her as a surrogate younger sister. With Kento, she shares a mutual, implied romantic connection, underscored by unwavering loyalty and deep concern for his well-being. Her rescue of Danji established foundational trust, with Danji expressing lasting gratitude for her lifesaving act. Throughout events, Sanae remains a steadfast, compassionate presence, emphasizing communal support and perseverance. Her background as a witness to profound loss fuels her commitment to protecting her found family at Adalus Base, her supportive role remaining consistent.