Known originally as Emilia Justina, Emi Yusa is the central heroine from the story who once served as the champion of the world of Ente Isla. Her background is defined by a mixed heritage, as she is the daughter of a human farmer named Nord Justina and an archangel named Lailah. Her mother abandoned the family due to the taboo of an angel bearing a human child, leaving Emilia to be raised solely by her father on a farm. This peaceful life was shattered when the army of the Demon King Satan destroyed her village. Believing her father to have been killed in the attack, a young Emilia accepted the call of the Church of Ente Isla to become the Hero destined to defeat the Demon King, her resolve fueled by this profound loss.

Emi's personality is a complex blend of determination, a strong moral compass, and a quick temper. Though initially motivated by a desire for revenge, she possesses a deep-seated benevolence and refuses to pursue vengeance against her targets when she sees they are not actually doing anything wrong. She is characterized by an unyielding resolve and a profound commitment to safeguarding innocent people. Her virtues include a heart unblemished by malice and steadfast allegiance to her close companions. However, she can also be impulsive and prone to exaggerated reactions, especially when dealing with the former Demon King. She also exhibits a notable self-consciousness about her physical appearance, particularly concerning her breast size, which she often compares unfavorably to other female characters like Chiho Sasaki. Despite her fierce demeanor, she can be quite clumsy in everyday situations and has a fondness for Japanese historical dramas and curry.

Emi's primary motivation shifts dramatically over the course of the narrative. Initially, her singular goal is to destroy the Demon King Satan to avenge her father's death and liberate Ente Isla. She follows him to modern-day Japan to finish him off. However, after observing the peaceful and mundane life he leads as Sadao Maou, she begins to question her mission. Later, critical discoveries undermine her original motivation: she learns that her father is still alive, having been secretly rescued and hidden on Earth, and that her mother has been manipulating events from the shadows. The loss of her desire for vengeance forces her to re-evaluate her role as a Hero and find a new purpose in protecting both Earth and Ente Isla from emerging threats, eventually leading to a formal alliance with her former enemy.

In the story, Emi serves as a primary foil to the protagonist, Sadao Maou. Her role evolves from that of a vengeful antagonist to a reluctant ally and eventual protector. Upon arriving in Japan and losing most of her magical powers, she assumes the alias Emi Yusa, registers as a 20-year-old adult, and finds employment as a call center agent to sustain herself while covertly monitoring the demons. She becomes an integral part of the group that faces threats from Ente Isla, using her knowledge and abilities to defend their new home. Her role also becomes intertwined with domestic life when she and Maou become the adoptive co-parents of Alas Ramus, a sentient fragment of the holy tree Yesod who takes the form of a young child and recognizes them as her mother and father.

Key relationships are central to Emi's character development. Her relationship with Sadao Maou is the most significant. They begin as mortal enemies, but as they are forced to cooperate on Earth, their dynamic shifts. She observes his genuine kindness and struggles to reconcile the caring human she sees with the cruel Demon King she once knew. Maou apologizes for his past actions, claiming he never truly understood humans, which further confuses her. As they co-parent Alas Ramus, they grow steadily closer, with Emi eventually developing romantic feelings for him that she struggles to accept. Her relationship with her coworker Rika Suzuki is one of close friendship and normalcy, as Rika is unaware of the supernatural world but offers support. Initially hostile to the demon Shirou Ashiya, their relationship improves over time to become more cordial. She is also protective of Chiho Sasaki, a human who knows the truth about their origins, though she sometimes feels jealous of Chiho's chest size. Her relationship with her estranged mother, Lailah, and her rediscovered father, Nord, also play crucial roles in reshaping her understanding of herself and her past.

Emi undergoes substantial development throughout the narrative. She transitions from a driven, revenge-fueled warrior to a more introspective and emotionally vulnerable individual. The revelation that her father is alive and that her vengeance was based on incomplete information forces her to question her entire identity as a Hero. Her time in Japan, observing the mundane struggles and kindness of those she once considered enemies, softens her rigidity. She learns to trust, to rely on others, and to form a family unit in the most unconventional of circumstances. Her emotional journey becomes more complex as she grapples with her lingering resentment, her growing affection for Maou, and her desire to protect the peaceful life she has found.

As for her notable abilities, Emi is a half-angel hybrid, granting her access to holy Celestial Force magic. Her powers include the ability to fly, create protective barriers, manipulate memories, induce sleep, and fire energy projectiles. Her signature weapon is the sacred sword Better Half, which is forged from heavenly metal and can assimilate fragments of the Tree of Life. Among her techniques are the Heavenly Flame Slash and the Heavenly Ice Dance. Upon arriving on Earth, the low magic levels in this dimension heavily limit her powers. To counteract this, she can temporarily replenish her magical reserves using a special concoction known as Holy Vitamin Beta. When she uses her powers at full strength, her physical appearance changes, with her hair turning silver and her eyes becoming red.