Hanzou Urushihara / Lucifer is a fallen angel and one of the Demon King's generals. After his defeat at the hands of Hero Emilia, he was secretly rescued by Olba Meyer, who promised to return Lucifer to heaven if they became allies in defeating the Demon Lord. After being defeated by the Demon Lord, Urushihara moved in with Sadao Maou and Ashiya.
Lucifer is shown to be cunning and sneaky as well as a childish, preferring to surf the internet and play video games rather than help Ashiya with the housework. Due to his attitude, everyone tends to treat him lightly and pay no mind to many of his protests at being excluded from things. Lucifer is also self-proclaimed socially awkward, lazy to an extent, and has a short temper. However, it seems he has hidden feelings beneath his seemingly irresponsible and carefree attitude, and shows his serious side from time to time. To his irritation, Lucifer is usually referred to as a hikikomori even when among company.
Known as the "original sin" and the son of Ignora, Lucifer was born after the destruction of the angels' home-world, making him part of the second generation of his kind. Once an honored and revered archangel, he was tasked with the responsibility of leading the archangels. Lucifer eventually abandoned his job, feeling it was too boring in heaven for him to remain. Losing his former status and gaining black wings signifying his corruption, his powers became demonic, reclassifying him a fallen angel. Lucifer traveled to the Demon Realm where he would remain for several years. During his time among the warring demonic tribes, Lucifer gained a certain amount of power and influence over his new compatriots, though was still not considered a "pure" demon due to his celestial origins. He would later join the young Satan Jacob in his plot to unite the demon realm under his flag, the rising monarch proclaiming that Lucifer should feel free to kill him if he ever felt displeased with his leadership.
(Source: Hataraku Maou-sama Wikia, edited)