The most devout and loyal of Satan's followers, Alciel was the leader of a major demon faction when they first met, and became his first major ally in their quest to unify the warring tribes of the Demon World. Upon establishing a centralized government with Satan as their supreme leader, the newly christened Devil King appointed Alciel to be one of the four Demon Generals during the razing of Ente Isla, acting as chief strategist and most trusted adviser to his Demonic Highness. As a result, the two have forged a strong bond through battle and conquest, with Satan trusting in Alciel's judgment while the latter faithfully protects and executes his master's oders.
While Satan originally viewed him as a servant, their relationship has since become more familial than hierarchical due to their self-imposed banishment on magic-deprived Earth. Stripped of their superhuman powers and most of their pride, the two demons were forced to rely on each other for survival in modern day Tokyo, living an impoverished life together in an apartment dubbed the "New Devil's Castle". In this new life, Maou trusts Ashiya to handle the household chores and finances while the former Demon King works to support their living expenses. Due to his subordinate's keen intelligence, he also assigned his former general the task of searching for sources of magic on Earth. Despite their former status as master and servant, Ashiya often lectures the Demon King on his sloppy style of dress, unhealthy habits, and excessive spending. This irritates Maou to no end, who feels as though his roommate has devolved into a troublesome parent. In spite of these changes in their relationship, Alciel still holds his liege in high-esteem and tends to heep praise onto his master even in public, even referring to him as "your highness" in mixed company, greatly embarrassing Maou. However, the two share a somewhat brotherly bond as they rely on each other constantly while on Earth.
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. Satan first became aware of her existence during the invasion of the Eastern Continent, hearing that Lucifer (Urushihara) had suffered defeat at the hands of a Hero, and his forces eradicated. Though he originally saw her as a relatively small threat and deemed that the remaining generals could put an end Emilia's resistance, he was forced to reconsider upon learning that both Malacoda and Adramelech had fallen as well. He then reluctantly acknowledged her as a viable threat, and at Alciel's insistence, recalled forces from the East to fortify the Central Continent, turning their campaign into a defensive one. 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, he was surprised when she revealed her identity, though remains relatively impassive until she pulls a knife out to kill him. 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, even displaying pity upon learning that she was all alone in Japan.
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 he interacts with Emi, Maou has grown mostly relaxed around her, almost as though their enmity never existed.
The dynamic of their relationship 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 be her "papa" and Emi her "mama". 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, and begins to question his true motivations. He also refuses to tell her the real reason behind Ente Isla's invasion, as he is worried this may cause her more confusion and make her lose her grounding as a Hero.
While Emi is held hostage in Ente Isla, she finally accepts that Maou's personality as a human is not a ruse and he is truly a kind and caring person at heart. Although she cannot fully forgive him because his invasion had tragic consequences for the people of Ente Isla, the anger that was driving her outward animosity towards him is no longer there. After being rescued by Maou and company, Emi's demeanor with Maou changes drastically - she is not hotheaded but calm, openly shows her gratitude to him, and treats him more like a friend than an enemy. Maou is extremely shaken by her radical change in behavior and does not know how to interact with her. He attempts to create rises in her while surreptitiously aiding her, but she sees through all of it and knows that he is trying to hide his kindness behind harsh words and actions.
Maou becomes extremely concerned for Emi's mental health following her return from Ente Isla and the arrival of her long-lost mother who has secretly been pulling the strings from the shadows. As Emi experiences a mental breakdown resulting from this, she openly clings to Maou for support in spite of how her actions might be misinterpreted. Over the course of the Ente Isla incident and the events afterwards, it becomes clear to others that Maou truly cares about Emi. However, when confronted with this information, he becomes extremely defensive and does not want to admit it.
While it is unclear whether Maou and Emi are harboring the beginnings of romantic feelings towards each other, it is worth noting that there have been hints of some level of attraction throughout the LNs. At their initial meeting in Japan when they did not know each other's identities, Emi's radiant smile made the dark feelings in his heart fade away and he felt awkward when he left the scene. This smile has a lasting impact on him, and he is reminded of it again during an event following Emi's rescue from Ente Isla. During the confrontation with Lucifer and Olba, Emi is shocked when she realizes that she found the smile on the newly restored Devil King's face to be charming/adorable. When Maou offered to show Emi a new world after naming her a Demon General, he found her flushed expression to be very cute and wanted to see more of it. After Emi shows him a smile after seeking comfort from him, he feels that her smile is precious but does not know the reason he feels that way.
A part-time co-worker at MgRonald's. The two are on very good terms
Maou is a very friendly and helpful superior to Chiho, which baffled and surprised Emi at first. He also calls her 'Chi-chan', signifying their closeness with each other. During the incident with Lucifer, she gained knowledge of Ente Isla as well as Maou's true identity.
Chiho has a very obvious crush on Maou, but he has yet to respond to her feelings despite her confession. When Maou asked her how she felt about associating with a Demon Lord like him, Chiho stated that she would be lying if she said she didn't mind, but she already had feelings for him before she knew of that fact, and wants to choose to stop liking him herself. Maou was surprised upon her bold confession, but when he tried to answer, they were interrupted.
His landlady, who insists on having Maou refer to her as Miki-ti. Her presence somehow intimidates him, as well as Ashiya and Urushihara. She seems to have an idea of Ente Isla, as she has knowledge of Sonar and the Idea Link. However, the reasons behind her understanding are still unknown.
His boss at MgRonald's. The two have the usual boss-and-worker relationship. Maou is seen to be fearful of her at times, but Kisaki looks upon him as a capable employee and is able to entrust him with the responsibility of supervising the restaurant in her absence.
Lucifer is a fallen angel and one of the Demon Lord's generals. After being defeated by Maou in Japan, he moved in with Maou and Ashiya to hide him from the police and help research magical sources with his internet knowledge. He often spends Maou's money unwisely on online shopping and has caused Ashiya many headaches with the household finances. Despite this, Maou doesn't seem to mind, treating him the same as he always had. However there were still exceptions, depending on how far Lucifier had gone.
Aside from Camio, Lucifer has known Maou for the longest, since Maou was still a small goblin.