Mayuka Yoshizumi is alien royalty forced to flee her home planet with her mother and bodyguard after a violent coup targeting her family. On Earth, her critical mission is to locate and reactivate the Ultimate Weapon to counter those who overthrew her dynasty.

She possesses yellow or dark blonde hair and cyan or blue eyes. Her height is 153 cm, with body measurements of 73-53-80 cm, and her blood type is AB.

Her spacecraft crash-landed on college student Yoshizumi's roof. During their first encounter, he seized her specialized weapon—designed not to injure but impose brainwashing scenarios involving forbidden relationships—and fired it at her. This implanted false memories, making her firmly believe she is his younger sister, fulfilling her deep-seated desire for an older brother to genuinely care for and love.

Personality-wise, she embodies a modern tsundere archetype, often brash and combative. Though aware her feelings for Yoshizumi stem from artificial brainwashing, she experiences internal conflict about their authenticity. This conflict doesn't significantly alter her outward behavior; she frequently annoys Yoshizumi, disrupts his daily life, and persistently tries to retrieve her weapon.

Her mission centers on awakening the powers of the red-haired, green-eyed Ultimate Weapon, Akiho. Activation requires applying "primitive stimulation," a process tied to observing and replicating human mating behaviors. Despite initial resistance, Yoshizumi eventually collaborates with her to achieve this objective.