Normally, the Tree of Life Yggdrasil would dwell in a reality different from normal humanity, but every once in a while it would make its presence felt in the mortal world, though invisible to humans, and bloom fruits onto the world. These fruits are invisible and normally float through humans harmlessly. They take imprints of human memories with them, disappearing into the ground to return to Yggdrasil with the memories. But in the clusters of fruits Yggdrasil lets loose, several special fruits would float off as well. These are Time Fruits, and the Time Fruit's effects will vary.
When a time fruit passes through a female, said female would become an immortal, this is not limited to humans as shown with Rin’sand Mimi’s immortal pet dog. However males will turn to Angels.