The goddess of Chaos uses one of her minions to summon an Isakai'd soul to this world as the new demon lord. Lacking physical power, the character learns more powerful magic as they build up their dungeon with monsters as they get more power.
The first girl is the one who summoned you and acts as your second in command. All the other girls are adventurers who attempt to destroy the dungeon as the Demon Lord gains more power. Their hair color changes when they have been infused with enough dark energy to become servants of the Demon Lord.
4 of the later girls are dlc characters. Too dangerous to mind control but too useful to kill, they instead are given to your scientist to experiment on. End result is that they can give birth to high level / strength but short lived goblins.
The last three are civilians for some of the monsters. If they give birth to a monster once, they can never give birth again The last one suffers a particularly brutual fate.