Sarina Shizukume starts as a seemingly welcoming middle school student who aids transfer student Aya Asagiri when teachers mock her. She interprets Aya's shyness and selective mutism as ingratitude, fueling resentment. This escalates into severe bullying with friends Ai and Erika Kaijima. Their actions encompass physical assaults, stuffing garbage into Aya's shoes, hiding razor blades in her belongings, waterboarding her in restrooms, and orchestrating an attempted rape by Shota Arai. Sarina's behavior stems partly from her own insecurities and susceptibility to peer manipulation, especially from Erika, who mocks her initial kindness. Her home life involves an absent father—implied due to divorce or death—and a mother and sister, with sparse details. After Erika and Shota die from Aya's defensive wand use, Sarina confronts Aya in a school bathroom, intending to mutilate her with a box cutter. Tsuyuno Yatsumura intervenes, using time-stop abilities to slit Sarina's throat, leaving a permanent scar. Hospitalized, Sarina overhears discussions about magical girls. Unlike most who gain powers from preexisting suffering, Sarina receives her wand directly from administrator Nana, driven by revenge and tasked with replacing the comatose magical hunter Rina Shioi. Her primary wand is a yo-yo (emblem: Φ) generating razor-sharp threads capable of slicing physical objects; its range is initially limited by thread length. Using it causes blood to flow from her ears and turns the tips of her red hair yellow. She later demonstrates improved control, extending the thread to swing across cities like a grappling hook. Temporarily, she wields Aya's teleportation gun and Mikado Ikemata's force-field pen wand, the latter used to trap an administrator in a self-destructive attack. As an antagonist, Sarina pursues Aya and Tsuyuno to avenge Erika and Shota, hypocritically blaming Aya while ignoring her own role in provoking the deaths. She coldly mocks Aya's trauma, including the death of her cat Mya. Sarina beheads a crow accidentally during power testing without remorse, reveals Rina Shioi's location to vengeful Nijimi Anazawa to provoke conflict, and kills Tsuyuno's family murderer, Soji Odano, primarily to antagonize Tsuyuno. During a battle in Tsuyuno's apartment, her recklessness collapses the building, killing 19 residents and injuring 39 others; she expresses concern only for Aya and Tsuyuno's near-fatal injuries. Sarina's character shifts after alliance-forming events against the administrators. She acknowledges her past cruelty, influenced by Aya's persistent kindness and self-sacrifice. She offers her remaining lifespan to Aya to aid the group's survival, partners with her in battles, and expresses explicit regret for her bullying. Her development culminates in self-sacrifice: during the final conflict, she exhausts her lifespan using the force-field wand to contain Administrator Juu's attack, dissolving into black mist in Chapter 126. This act, alongside her earlier protection of her sister from administrators, underscores her redemption. Her fate remains ambiguous post-Tempest, though administrators imply deceased magical girls may reincarnate as new site managers. Physically, Sarina has fair skin, red eyes, and red hair typically tied in pigtails with left-sided bangs. The neck scar from Tsuyuno's attack is persistent. Her personality evolves from explosively vengeful, envious, and hypocritical to protective and selfless, retaining some abrasiveness early in her alliance phase. Hobbies include shopping, photo booths, sports, and movies; she enjoys pools, carbonated water, and doughnuts but dislikes studying and coffee. She also expresses disdain for indecisiveness and cats, hinted when insulting Mya.