Kanojo Shoukan Shimashita!?

A high school student with no romantic experience finds himself intrigued by a classmate, unsure if his feelings amount to love. Meanwhile, his younger sister, who is fascinated by summoning demons, conducts a ritual that unexpectedly summons a girl named Lars from the underworld. She presents him with a contract, setting off a comedic romantic entanglement involving seven people. The story introduces a condition where infidelity could lead to the end of the world.