Satomi Nishiyama, known as "Limit," bears this nickname from a plane crash that claimed her mother's life and left her mortally wounded. Her father, Dr. Nishiyama, saved her through cybernetic reconstruction, making her a cyborg. This traumatic event and her resulting cybernetic body form her core background, fueling ongoing emotional struggles with identity and a fear of rejection due to her nature.

Her cybernetic abilities activate primarily through a special pendant. These include "Miracle Power" granting superhuman strength, "Miracle Jump" enabling enhanced leaping, and "Miracle Run" providing extreme speed. She also utilizes seven specialized tools: a winged shoulder bag for flight, a red beret serving as a communication device to contact her father, red boots bestowing dancing prowess, lip balm for writing messages, a light-emitting compact, a coin purse that repels thieves, and a flower ring emitting a hypnotic, calming fragrance. The pendant allows her to alter her appearance using the phrases "Change Face" and "Change Back," typically for disguise or assuming an adult form. A proposed limitation that excessive power use would shorten her lifespan was not retained.

She maintains key relationships: her father, who researches methods to restore her humanity; Midori Yumoto, his assistant and one of only two people aware of her cyborg identity; Nobuko Asami (Buko), her closest human friend, though their friendship faces strains from jealousy or misunderstandings; Boss (Ryūta Ishibashi), a class bully whose antagonism often drives conflicts; Mitsuko Takeshita, a rival student challenging her social standing; and Guu, a solar-powered robot dog replacing a pet lost in the crash, serving as her companion.

Her development centers on profound insecurity about her cyborg identity and the necessity for secrecy. Episodes depict her grappling with isolation, avoiding romantic feelings due to her inability to age normally, or experiencing brief estrangement from her father over perceived rejection. A recurring theme involves resolving interpersonal conflicts non-violently using her powers while ensuring her actions are attributed to others, often Boss, to maintain secrecy. Her character arc culminates in a pivotal act of self-acceptance when she deliberately reveals her true nature to her teacher to save her life. Though the teacher agrees to secrecy, this act signifies her growing willingness to confront the potential consequences of her identity.

The series occasionally revisits the memory of the plane crash and her mother's death, reinforcing the emotional weight of her transformation. While focused on school life and friendships, these everyday challenges consistently intersect with her efforts to reconcile her humanity with her mechanical existence.