Touka Satomi is an 11-year-old magical girl and former patient at the Satomi Medical Center in Hokuyo Ward, directed by her father. Confined due to severe childhood illness, she cultivated exceptional intellect in astrophysics and technology, often seeking refuge in astronomical studies to escape isolation. Her early environment prioritized academic achievement over emotional connection, fostering a self-centered worldview where knowledge was personal property not to be shared. Hospitalization led to a close friendship with fellow patients Nemu Hiiragi and Ui Tamaki, though Touka frequently argued with Nemu over Ui's attention. Iroha Tamaki, Ui's older sister, became a protective figure Touka viewed as family. The four shared a bond Touka considered familial.

Touka, Ui, and Nemu discovered the truth about magical girls' inevitable transformation into witches. To save Iroha from this fate, all three made a collective wish contract with Kyubey. This inadvertently triggered Ui’s absorption of impurities from other magical girls. Forced to preserve Ui, Touka and Nemu transferred her consciousness into a modified Kyubey unit, erasing Ui's existence from global memory—including their own. This memory loss fundamentally altered Touka; devoid of Iroha and Ui’s positive influence, she descended into deepening arrogance and detachment.

As co-founder of the Wings of the Magius alongside Nemu and Alina Gray, Touka spearheaded efforts to "liberate" magical girls from their witch-bound fate. She developed the Doppel system to bypass witch transformations and deployed brainwashing Rumors, trauma, and sacrificial tactics to achieve Magius goals. She justified these methods through utilitarian logic, dismissing ethical concerns. Her magical ability, derived from her wish to "take the power to [convert] from Kyubey," enabled near-limitless energy conversion without soul gem corruption, granting exceptional combat power. She wielded a parasol for flight and energy projection.

Touka’s Doppel, Pennen Nolde, embodies her suppressed desire for greater power. Modeled after "The Little Match Girl," it divides her lifespan into matches; each lit match grants a wish at the cost of her life. Its form references her hospitalization and unattainable hopes. After Iroha restored Touka’s memories of Ui, Touka realized Eve—the entity central to the Magius' plan—was Ui. She aided Iroha in rescuing Ui and halting Eve's rampage. Post-conflict, the Rumor of the Eternal Sakura tried Touka and Nemu for their crimes. Accepting culpability, Touka received a sentence forbidding magical girl transformation under penalty of death. Though she retained her belief in magical girl supremacy, she declined joining the Neo-Magius to avoid causing Ui and Iroha further distress.

Later, Touka volunteered at her father’s hospital, teaching astronomy to pediatric patients. Initially faltering at simplifying complex concepts, she gradually acknowledged the value of sharing knowledge to inspire others facing illness, marking tempered growth from her earlier selfishness. Her dress design, resembling prison bars, symbolizes her lifelong confinement: first physical, then ideological.