Back in Ente Isla, they were mortal enemies. Emilia's village had been destroyed by the Demon King's forces, and her father was presumed killed. In order to take him down, she took up her duty as the Hero and trained harder than ever, rallying forces against the dark legions and leading them into battle. Emilia admits later on that she had been obsessed with the idea of revenge, relishing the feeling of battle as she sliced off one of Satan's horns, and chasing him through the Gate when he fled Ente Isla. It is his defeat at the hands of the hero that forces the Demon King to retreat to Earth, resulting in the subsequent loss of his powers and much of his pride.
They did not recognize each other at first when they met again on Earth, she discerns his identity sometime later and openly confronts her nemesis in the streets, pulling a knife on him in an attempt to finish the tyrant. Strangely he does not fight back and instead chooses to run, the scrap ending in a draw when the police pull them aside and assume they were having a lover's quarrel. While she begins to dedicate her time to spying on him in case he reverts to his demon ways, Maou remains more irritated by her presence than anything else, and despite her long hours of surveillance, she can find no trespass in his actions. Growing tired of watching him, she eventually questions whether or not he even intends on returning home, though the Demon King claims he will subjugate both Ente Isla and the Earth one day, prompting her to stick around and keep an eye on him.
Though he still expresses a desire to return to Ente Isla and reclaim the lands there, he shows very little antagonism or ill will towards the Hero, and even begins showing her kindness and concern. This is first shown when they help each other escape an assassination attempt from an unknown assailant, and later when he orders Ashiya to bring the Hero some medicine and personally treats several of her wounds. Emi eventually confronts him over this change in personality, noting he is much different from the Demon King she fought back home. When mourning the losses she suffered when his army destroyed her home, Maou admits that back then he never truly understood humans, and genuinely apologizes, much to her confusion. Due to the loss of her father, Emi viewed Maou as a despicable person who thinks nothing of creating destruction and bloodshed. As such, seeing his kindness shocked her extremely, for she could not reconcile her idea of the Demon Lord with his human self.
While they still remained a little antagonistic to each other, the two eventually came to a truce. They have since come to understand each other better, and regularly join forces to combat the continued emergence of threats from Ente Isla that pose danger to their new home and friends. On occasion they both acknowledge that the day may come when both will be enemies again, though as the story continues, this becomes less apparent. Due to how often she interacts with Maou, Emi has grown mostly relaxed around him, much to her surprise. At time it is almost as though their enmity never existed. The dynamic of their relationship first improved when they began taking care of the sentient Yesod fragment, Alas=Ramus, who took the form of a young child that believes him to her "Father" and Emi the "Mother". Since then, the two have steadily grown closer while being forced to act as a family, with both slowly falling into the roles of parents over time.
After learning that her father is still alive, the Hero loses much of her previous bitterness towards the Demon King, her emotions towards him becoming increasingly conflicted. She eventually develops romantic feelings for Maou due to the time they spend together, eventually falling in love with him. Over time she begins to see him as a reliable comrade and a strong ally, and due to their encounters with dangerous assailants from Ente Isla, has been saved many times by her former enemy. After being rescued by Maou during a return trip to Ente Isla, she begins to express her growing fondness for him openly, much to the happiness of Alas=Ramus and Chiho's jealousy. Emi's gentler disposition and more recent displays of affection are shown to confuse Maou, prompting him to react awkwardly when experiencing her kindness due his lack of familiarity with this side of her. It is unknown if he returns these feelings to the same extent, though Lailah has noticed how close the two have become, noting that the Demon King spends a lot of time watching Emilia, and appears to understand her better than anyone else.
His actions are dense with how she feels sometimes, as Emi has yelled at him for reasons beyond his comprehension. There are times when he gets frustrated with Emi, but also shows no hesitation in riling her up, saying that the Demon King is supposed to cause trouble for the Hero. Nevertheless, Maou still enjoys teasing her whenever possible, and seems to find her reactions amusing.
During their first encounter, Emilia fought with the Demon General Alciel, driving him into a retreat to the Demon Lord's Castle on the Central Continent. While having been mortal enemies back in Ente Isla, they share a cordial acquaintance on Earth, speaking to each other civilly despite a certain sense of dislike still lingering between them. Due to her hatred of the Demon King's past exploits back home, Emi does not hesitate to put down Maou in front of Ashiya, which aggravates him to no end. In addition to this, she also finds his frugal nature to be quite irritating, often pointing out his tightfisted habits, or exploiting them in order to coerce information from him.
Her friend and colleague at Dokodemo, Rika appreciates Emi for not prying into her past when the former revealed that she was originally from Kobe. Rika is usually seen hanging out with Emi at work and in their free time. After Emi refused to join her for dinner after work, Rika became curious about her friend's personal life, eventually finding out about the hero's hostile relationship with Sadao Maou. She initially assumes the two are dating, much to Emi's horror, but her investigations into the topic eventually Rika to meet Ashiya, who clears away the misinterpretations with a story of his own. While highly supportive of her friends, Rika has a tendency to pry into Emi's personal business, though the latter takes it in stride, never holding it against her.
They did not get off to a good start, as on their first meeting Emi warned Chiho about Maou's character and unwittingly mislead Chiho into thinking that she had a relationship with Maou. Now, she is on friendly terms with Chiho, the only human on earth that knows the true identity of the people from Ente Isla. At times, she gets jealous of Chiho's chest size.