Use the number 13 as a plot device. Is she the 13th failed clone? The 13th zodiac? The daughter of the 13th moon? Revealing this truth should be a major turning point in the romance (e.g., "I wasn't made to love; I was made to kill... until you.")
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A rare but fascinating sub-genre: Animal Girl 13 is not the pursuer but the prize —and she rejects that notion. In a reverse harem, multiple suitors (a hunter, a scientist, a prince, a monster) vie for her. Use the number 13 as a plot device