Emi Yusa / Emilia Justina is the main female protagonist and the Hero who defeated the Demon King Satan. After forcing the Demon King to retreat from Ente Isla, she followed him to modern day Japan to ensure his destruction. However, just like the Demon Lord, she lost most of her powers upon her arrival and was forced to assume a Japanese name. She was also forced to find employment, and so became a call center agent. She is a half human and half angel.
Overall, Emi is a strong-willed and determined hero, displaying many admirable qualities befitting of one in her position; among which are a pure heart, boundless courage, unwavering loyalty to those closest to her, and a deep-seated desire to protect the innocent. Moreover, she lacks self-confidence and gets angry/frustrated easily. While initially hesitant to become the great hero of Ente Isla, the death of her father at the hands of Lucifer's Army ignited a fire in the heart of the young hero, giving her a motivation to train harder and strike down those who destroyed her village. Due to her personal losses, she sees all demons as her enemy, most of all their leader, Satan Jacob.
Emilia Justina was born to her parents, an ordinary farmer named Nord Justina and the archangel Lailah. Knowing she had committed a taboo by birthing a child to a human, her mother fled, leaving Emilia to be raised by her father on his farm located somewhere on the Western Continent. During Satan's conquest, the child lived a peaceful life with her father. Despite fearing the day they would be accosted by the encroaching demon army, he assured her that their village would be protected by the Church, putting her mind at ease. Her life changed drastically one afternoon when representatives from the Ente Islan Church arrived at her home to train Emilia as the Hero who could free the world of the Demon King Satan. Although reluctant at first, Emilia's motivation was fueled when she received news of her father's apparent death when the Demon King's army laid waste to their fields only days after her departure. In reality, Nord Justina was taken by Lailah and hidden safely on Earth during this time, though Emilia remained unaware of her father's true fate.
(Source: Hataraku Maou-sama Wikia, edited)