Maou-jou no Nisemono-hime: Shujin no Migawari ni Totsuida Kyuuji-gakari ga Shokei Kaihi wo Mezashite Hisshi ni Nattara Maou-sama ni Kanchigai Sarete Dekiai Sareru Ken

As a substitute for her mistress, the princess, the maid Elly is to marry the Demon King. If her true identity as a fake princess is discovered, she will be killed by the demons...! She plans to endure for just three years, acting in a way that will keep the demons at bay, but... The more she tries to make herself disliked, the more the Demon King begins to dote on Elly. Is this love that began as a substitute real or fake...? (Source: Kodansha, translated)