nyanyan00's manga / #ch.16(16)

she pretended to be the duchess while the Duke was away for the war. She did this cuz she wanted to get away from her stepmother and step little sis

the real female lead (rosalin) is already dead. she died from falling off a cliff. before she died, a demonic mist or being of some sort that only eats dead things approached her as she was close to dying already. Rosalin notices this black mist and asks it to grant her wish, which is to protect the 2nd prince, then she'll give her body to it. the black mist agrees and devours her or possess her body. so the rosalin now is called the shadow of rosalin and its mission is to protect the 2nd prince.