it isn't really the emperor's fault, basically everyone has no memory of the girl existing due to her being a half-blood. if you remember at the first chapter there was something about the blessings of god being bestowed on the emperor's family. there was another "race" or group (idk what too call them exactly) who worshipped another god, and was bestowed the blessings of that god. how the gods work in this world is basically the more people believe in them, the more powerful they are. unfortunately, only MC's mum is the sole remaining "pure blood" of said race, and when she died (i think during child birth) no one was left to remember the god, nor the little daughter she had.