Our main character finds himself reincarnated into a fantasy world full of elves, dwarves, orcs, and magic. He sets out to create a harem for himself, just like any isekai protagonist should. Unbeknownst to him, Ruru, the demon maid he summoned has other plans.
She wants the MC for herself.
In order to accomplish this she will seduce and conquer any women that tries to get between them. Or adjacent to them. Or who just so happens to be in the vicinity. She's not picky.