He Who Vowed to Create a Demon Harem

What happens when a hero falls for the demons he’s supposed to destroy?
Strange circumstances bring a human hero together with three demons,
and the four of them set out on the adventure of a lifetime.
They don’t work well together at first, but after a while (and with a little help from a special item),
these old enemies will start to REALLY get along…The year is 1887.
Humankind has been assailed by demonic invaders from hell. For a time,
humanity’s future was uncertain… until brave soldiers arose who fought back the demons. These soldiers are called heroes.
One such hero, a man named Theo, has a goal in mind: take back the city of Manc from the demons who control it,
the Halfas family. But the daughters of said family ended up being a lot cuter than he expected…​