In the clusters of fruits Yggdrasil lets loose, special ones called Time Fruits float off as well. If a female, human or otherwise, were to encounter one of these Time Fruits, they would bond to the female and make them immortal, and would only be aware of this if they were to suffer a violent death. However, if a male were to be infected, they become creatures Rin would later call "angels", although there is nothing heavenly about them. These creatures are short-lived and violent, and are drawn to the female immortals physically – and sexually, and the female the same. But the males, in the act of mating with them, will eat the females, hoping to get to their Time Fruits. And that is why the females must kill the angels before them.