A long time ago, there existed a small village in the kingdom of Lugunica inhabited solely by Oni, human-like demons with the ability to grow horns to reach superhuman levels of power. That village was home to many prodigious warriors, but none were as peculiar as a particular set of twins. Ram and Rem were born with a single horn each, unlike all other Oni, which have two horns. The older twin, Ram, was an extremely talented and powerful magic user. Rem, however, was more on the clumsy side, always depending on her sister.

One day, Rem realized that she needed to be more helpful in order to become as talented as Ram, but that didn't go very well, as she got lost in a forest when trying to find ingredients for a dish, only to be rescued by Ram. Despite her frustration and inferiority complex, Rem still loved her sister more than anything else, and she looked forward to learning more about the world with her help. But then, tragedy struck. A grim organisation known as the Witch's Cult invaded Rem and Ram's village and brutally murdered all of its inhabitants, except for Rem and Ram, who were able to fight them off with their powerful magic. However, during the fight, one of the cultists sliced Ram's horn off, rendering her much weaker. Rem secretly felt joy at that moment, knowing that from that point onward, she was going to be the superior sister.

Soon after the destruction of their home, the twins were taken in by an influential man known as Roswaal L. Mathers, one of the most powerful mages in Lugunica, and owner of large portions of land. The twins were allowed to stay in Roswaal's manor in exchange for them working as maids with the assistance of the chief maid, Frederica Baumann. During their time working at the Roswaal Manor, the twins were assigned to take care of Emilia, a candidate for the Lugunican crown. This alliance also lead to them meeting a strange human known as Subaru Natsuki.