Encyclopedia Entry:
A monster that is thought to be born from a fusion of the lingering regret of those who lost love, and the monster’s demonic energy. They reveal themselves after being drawn by the lingering regret of men who were rejected by the women they loved. They don’t have a fixed form. Instead, they read the minds of human men, and appear in the form of a woman that the man had a one sided crush on. Not only do they look like her, they also take on the woman’s memories, personality, and abilities, making them their own.
However, their form is modified to be based on the memories and ideals of the man who was targeted. For example, even if the woman a man had feelings for was “nice on the surface, but malicious and scheming on the inside,” she becomes a woman matching the man’s ideal that is “pure and gentle, even in heart.” Also, since the doppelganger herself has feelings for them man, the reason they transform is “to become the man’s ideal woman so he can’t help but love her.” Most men aren’t able to resist the temptation of a doppelganger, and they’ll have sex and release spirit energy on the spot when seduced. Even afterwards, they continue to be the man’s ideal lover, and continue to offer love and pleasure to the man.