Sadao is Ashiya's lord, to whom he has vowed eternal loyalty. Whilst not much is shown about their relationship in Ente Isla, on Earth they have a close friendship, often treating one another as brothers. Sadao even said that their transition to Earth was easier as it would have otherwise been because they had one another.
Ashiya is extremely dedicated to his Lord, usually referring to him as "sire" or "my liege". He will follow his every order to the letter, and becomes very angered if he witnesses anyone not giving Sadao the same amount of respect he himself does, not hesitating to yell at Emi when she insults him. He tends to become very upset if he feels like he has failed his Lord, beating himself up during such times and apologising loudly and proufously, often in public, much to Sadao's embarrassment.
Although he is extremely dedicated to Sadao, Ashiya won't hesitate to reprimand him at times, frequently nagging him in way that is reminiscent of a fussy parent, mostly due to his irresponsible spending habits, eating habits or dressing style.
Alciel and Lucifer both served as Satan's Demon Generals. Following Urushihara's failed attempt to murder the Devil King and the Hero, he now lives with Sadao alongside Ashiya.
Ashiya tends to view him as a nuisance, showing little respect or patience with him. He is generally seen nagging Urushihara for his laziness, lack of proper respect towards their Lord and, after he gained a computer and internet life, for his habit of spending more than they can afford on expensive luxuries.
At the start of the series, Ashiya had a tendency to freak out whenever he came into contact with Emi, going to great lengths to keep her from causing trouble for Sadao.
As time went on, they developed a cordial acquaintance, though Ashiya prefers to avoid Emi if possible. However, this changes if she insults his master. He has frequently stated that the latter is very benevolent towards Emi, and often outwardly scolds her for insulting him. This was best displayed when Maou sent him to sample products at Sentucky Fried Chicken; upon hearing Emi insult his Lord, he instantly approached her in a rage and told her to take back all she had said, speaking of how great Sadao is.
Following the words of encouragement Rika provided him regarding his meager role in comparison with his previous one, Ashiya seems to see Rika as a friend. In the light novel, the two went out a couple of times and exchanged phone numbers; however, Ashiya seems to be somewhat dense when it comes to girls and as such is oblivious to Rika's obvious crush on him, something even Maou picked up on.