Tsumugi Shiraui is a Gauna-human hybrid bio-weapon created by the scientist Ochiai using Kunato Developments' resources. Her creation involved impregnating a placental clone of the Gauna-assimilated Shizuka Hoshijiro with a human embryo, resulting in a chimera designed to combat the Gauna. Initially confined within a hangar due to her size, comparable to a Garde mecha unit, she interacts using a probe-like extension resembling a single-eyed worm with tentacles, capable of navigating Sidonia's ventilation systems. This probe can mimic human forms, analyze object composition, and probe unconscious minds. Her primary body is humanoid with a placenta-covered face resembling a mask, revealing teeth and one visible eye capable of expressing emotions. Despite her destructive capabilities, Tsumugi exhibits a gentle, childlike curiosity and playfulness, particularly around Nagate Tanikaze, Izana Shinatose, and Yuhata Midorikawa. Her personality may reflect residual traits from Hoshijiro, though this remains unconfirmed. Her combat abilities include kabi claws for penetrating Gauna cores, placenta-based beam weapons fueled by self-generated Heigus particles, accelerated regeneration, and sensory perception matching Sidonia's long-range scanners. She is vulnerable to Heigus particle depletion. Her relationship with Nagate evolves significantly. During a mission to Lem VII, they share a beach outing where Nagate confesses his love, accepting her non-human form and size difference. Overwhelmed, Tsumugi faints, evaporating seawater upon contact. She later discovers Kanata, a younger, unstable chimera sibling created by Ochiai, but is excluded from Kanata's escape with Ochiai. In the Battle for Lem VII, Tsumugi's fleet is destroyed by a Gauna Higgs particle beam, though she survives. Later, confronting the Gauna Benisuzume mimicking Hoshijiro, Tsumugi hesitates due to emotional conflict, allowing Benisuzume to impale and drain her Heigus particles. Nagate rescues her. During their subsequent fight against Chimera Ochiai in Lem's star, Tsumugi sacrifices her body to shield Nagate from stellar heat, resulting in fatal damage. Her consciousness is transferred into a captured Hoshijiro-like Gauna placenta sample. She marries Nagate, settles on Lem VII, and gives birth to their daughter Nodoka. Hoshijiro's fragmented memories persist within her new body, though they remain indistinct.