There are men, women, old people, children in our society... They are collectively called human beings. Human beings have been regarded as a symbol of nobility and superiority throughout history, while other creatures are inferior. Among these lower creatures, there is another special group. In fact, they are not much different from humans. They can do everything humans can do, but we still call them lower creatures just because they have a special bloodline. Here, we call them "different species"
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The first part of the X (Unknown) series, it is a fantasy, long story, with some romance and some ambiguity, with the main line being romance between a man and a woman.