Satomi Nishiyama, nicknamed Limit, survived a fatal plane crash that killed her mother, sustaining mortal injuries that led her scientist father to rebuild her as a cyborg to save her life. This cybernetic reconstruction granted enhanced physical abilities but also left her haunted by her non-human nature and the constant need to hide her true identity.

She activates scientifically derived powers through specialized accessories: superhuman strength ("Miracle Power"), extraordinary leaping ("Miracle Jump"), and exceptional speed ("Miracle Run"). Her pendant triggers physical transformation with the phrases "Change Face" and "Change Back." Additional tools include a winged shoulder bag enabling flight, a red beret functioning as a communication device to contact her father, boots enhancing dance movements, lip balm for writing messages, a light-emitting compact, a theft-deterrent coin purse, and a flower ring emitting a calming hypnotic fragrance.

Her daily life involves attending elementary school as a fifth-grade student, navigating a desire for normalcy. She maintains a close friendship with her tomboyish, bespectacled classmate Nobuko Asami. Social challenges include rivalry with popular transfer student Mitsuko and conflicts with a boastful bully known as Boss. At home, she lives with Guu, a robot dog replacing a pet lost in the crash, and Ms. Tomi, a bilingual housekeeper providing childcare.

The nickname "Limit" originated from an abandoned storyline concept where her cybernetic form had a strict one-year lifespan, removed during development for being too dark. The nickname remained despite its original context fading. Her experiences often reflect insecurity about her cybernetic identity and a wish to be fully human, though she consistently uses her abilities to resolve problems non-violently for others.