Sayaka Miki is a second-year student at Mitakihara Middle School and childhood best friend of Madoka Kaname. Tomboyish and energetic, she possesses a strong sense of justice, often acting heroically yet impulsively without considering consequences. Her idealism extends to unrealistic views on love and self-sacrifice, heavily influenced by her infatuation with violinist Kyosuke Kamijo. Witnessing Mami Tomoe's death but overlooking the dangers, Sayaka became a magical girl to heal Kyosuke's career-ending hand injury, driven by her desire to help others. Her knightly blue magical girl outfit features an asymmetrical skirt, cape, and a fortissimo-shaped hairpin reflecting her love of classical music. Her primary weapon is a cutlass sword, and her soul gem is embedded in her abdomen. Her wish granted accelerated healing abilities for rapid recovery.

Kyosuke dating Hitomi Shizuki shortly after his recovery deepened Sayaka's despair upon learning her soul resided within her soul gem, rendering her physical body a shell. This revelation, coupled with perceived rejection, led her to refuse grief seed purification, resulting in corruption. She transformed into the witch Oktavia von Seckendorff, whose mermaid-themed concert hall barrier symbolized unrequited love and isolation. She perished following Kyoko Sakura's sacrificial intervention. After Madoka Kaname's universe-altering wish erased witches, Sayaka's soul was purified by the Law of Cycles. She appeared briefly in the new timeline to witness Kyosuke's violin performance, expressing contentment with her choices despite her death.

In "Rebellion," Sayaka entered Homura Akemi's barrier as an agent of the Law of Cycles. Retaining memories of past timelines, she deceived Kyubey by posing as a regular magical girl while aiding Madoka's mission. She intervened to prevent Homura's excessive reliance on time manipulation and later summoned Oktavia to battle Homura's witch form. After Homura transformed into a demon, Sayaka opposed her rewritten universe.

Spin-off media present varied characterizations: In "Oriko Magica," she allies with Homura to protect Madoka. "The Different Story" depicts her as more perfectionistic, with earlier magical girl activities alongside Mami. The PSP game offers divergent paths, including scenarios where she avoids witchhood through Kyoko's intervention or confesses her identity to Kyosuke. "Wraith Arc" shows her struggling with focus during wraith battles due to lingering feelings for Kyosuke, leading to conflicts with Kyoko. Her family includes an unnamed mother and an aunt running an inn near Kamihama City, mentioned in "Magia Record." Her core traits—idealism, devotion, impulsiveness, and eventual self-destructive despair—remain consistent across all appearances.