Mamono Lord's husband

The Demon Lord's Husband is a former human hero, now an incubus, who (as the name might suggest) is married to the Demon Lord and sire to the lilims. He was described as "the strongest of humanity" in Monster Girl Encyclopedia I.

Overview
The exact order of events that brought the Demon Lord and her husband together is somewhat unclear in the Monster Girl Encyclopedia books. At any rate, after the defeat of the former Demon Lord, in a very unusual turn of events the remaining monsters (who were expected to largely cannibalize each other once robbed of their foul leader) instead rapidly acclaimed a new Demon Lord, a succubus. One of the Chief God's heroes - who would generally be expected to deal with this development by lopping her head off - had been nurturing in his heart "doubts about why humans and monsters must continue to fight". On meeting the new Demon Lord, he stuck her with his pork sword rather than his steel one and they ended up married.

Incensed by the betrayal, and the Demon Lord's grand magic that turned all the monsters into monster girls, the Chief God descended from Heaven to annihilate her foe personally. Unfortunately for her, sex with a succubus is a positive feedback loop with regards to spirit and demonic energy, and the married couple had built up their power sufficiently that together they were able to wound and repel the Chief God. Although they won the battle, they didn't win the war; the present state of affairs is untenable in the long term. Hitting the Chief God over the head, or even conquering the world, won't bring the Demon Lord's plans to fruition; instead she needs to overcome the original Chief God's metaphysical laws that are preventing monster girls from bearing sons. For this, she needs yet more demonic energy power - and so her main strategy is fucking her husband all day every day in the Demon Lord's castle.

Being married to a physical god, and now possessing "extreme power even rivaling that of a god" in his own right, the Demon Lord's husband is not a typical incubus. "He deeply loves his wife, but he has a strong sense of justice, and a stubborn side to him. It is said that the minions get spooked from seeing how he fights with his wife". When not fucking for the purposes of metaphysical revisionism, both he and his wife are accomplished Magic Gem crafters.