Lucifer once served under Satan as one of his Demon Generals. He was thought to have been killed by Emilia, but showed up in Japan with Olba, scheming to kill both Maou and Emi. When he failed to do so, Maou took him in to prevent him from being discovered by the police. Maou provided him with a laptop so he could research magical sources via the internet. However, Urushihara tends to waste Maou's money on buying video games online, or buying random items. Maou seems to harbor some degree of friendliness towards him, even though Urushihara does not contribute to the Devil's Castle, and allows him to have some degree of freedom indoors.
Together with Alciel, Adramelech and Malacoda, Lucifer was one of the four demon generals appointed by Satan. After arriving on earth and encountering Lucifer again, Ashiya was shot by Lucifer when the latter attacked Japan, but was able to join the fight against him after being aided to recovery by Maou. After Urushihara moved into Castle Overlord, Ashiya tends to lecture him about not helping with the chores and also gets frustrated by Urushihara's online shopping habits that endanger their household finances.
Lucifer is Sataniel's son. After finding out about Ignora's actions, Sataniel took their son and fled from heaven.
Ignora is Lucifer's mother. She experimented on him while he was still in her womb which made him immortal.