Emi Yusa, born Emilia Justina, is the half-angel, half-human daughter of human farmer Nord Justina and archangel Lailah. Her mother abandoned the family due to the taboo of an angel bearing a human child, leaving Emilia to be raised by her father on an Ente Islan farm. When the demon army invaded and she believed her father killed, Emilia accepted the Church of Ente Isla's call to become the Hero destined to defeat the Demon King Satan, her resolve fueled by this loss. She later learned her father was secretly rescued by Lailah and hidden on Earth.

Leading the resistance as the Hero of Ente Isla, Emilia defeated demon generals including Lucifer, Adramelech, and Malacoda. She confronted Satan at his stronghold, severed one of his horns, and forced him to flee through a dimensional gate to Earth. She pursued him alone after her ally, High Priest Olba Meyer, failed to follow. Upon entering Earth, she lost most of her magical powers. Stranded, she adopted the alias "Emi Yusa," registered as a 20-year-old in Japan, and secured employment as a call center agent to sustain herself while searching for Satan.

In Japan, Emi discovered Satan, now Sadao Maou, working at MgRonald's. Initially intent on killing him, she observed his mundane life and gradual moral evolution, conflicting with his past atrocities in Ente Isla. Her vendetta was complicated when the angel Gabriel revealed her father was alive, undermining her primary motivation for vengeance and prompting her to question her role as a hero and her relationship with Maou.

Her hybrid physiology grants access to holy Celestial Force magic. Her abilities include barrier creation, memory manipulation, sleep induction, energy projectiles, and flight. Her signature weapon, the sacred sword "Better Half," crafted from heavenly metal (tianyin), can assimilate fragments of the Tree of Life (Yesod) and enables techniques like the Heavenly Flame Slash and Heavenly Ice Dance. On Earth, her powers are limited by low magic levels, though she temporarily replenishes them using substances like Holy Vitamin β.

Emi's personality blends determination, intelligence, and a strong moral compass with impulsiveness and a quick temper. She initially distrusted all demons, especially Maou, but gradually acknowledged his genuine kindness in Japan. This shift led her to prioritize protecting innocents over vengeance. She formed close bonds with humans like coworker Rika Suzuki and Chiho Sasaki, shielding them from supernatural threats. Emi adopted Alas Ramus, a Yesod fragment, forming a familial bond with Maou as co-parents. This softened her rigidity, though she remained sensitive about her physical appearance, particularly envious of Chiho's breast size.

Witnessing the Church of Ente Isla's corruption and power struggles after the demon retreat disillusioned Emi. She severed ties with all Ente Islan factions to seek her own justice. After losing her call center job, she joined MgRonald's, working alongside Maou and Chiho. Maou later appointed her a Demon General, symbolizing their uneasy alliance against greater threats like rogue angels.

Key developments include reconciling with Maou after he apologized for his past ignorance of human suffering, experiencing emotional turmoil upon learning of her mother Lailah's survival (though they remain estranged), and committing to defend Tokyo from celestial conflicts. Her character arc concludes with a focus on protecting her newfound family and home rather than returning to Ente Isla.