This is easily one of the most famous Japanese urban legends of all time. A woman in a trench coat and surgical mask approaches you. She will not leave or go. In fact, you cannot escape; she is too fast. She asks "Am I pretty?" (It does not matter how you answer.) She will then take off her mask and reveal
her face. She will ask again: "Am I pretty now?" This is the kuchisake-onna (slit-mouthed woman).
Answering "No"
➠ In the urban legend, she either kills you or mutilates your face.
➠ In the movie, she kills you. The end.
Answering neutrally
➠ In some versions of the legend, she gets confused and you may escape. In others, she cuts your face and leaves.
➠ In the movie, you die. Terribly.
Answering "Yes"
➠ The legend is split on this. She either leaves satisfied and you have saved yourself, or she follows you home and kills you there. This is because "pretty" and "cut" sound very similar in Japanese (kirei / kire).
➠ In the movie? You die.
Not answering
➠ Urban legend: she is confused and leaves.
➠ In the movie, you die. Again.
Asking her if she thinks you are pretty
➠ This is a part of the urban legend. You may be able to distract her long enough to escape if you ask.
➠ In the movie, this gets you a giant pair of scissors stabbed into your torso a few dozen times. I assume you have seen the pattern.
The kuchisake-onna in the movie is also shown to be somewhat immortal. If her body is destroyed, she moves to another host. The body is no longer recognizable as anyone but the woman; it takes on her face and literally becomes hers. For this reason, you pretty much will not be able to tell this is Mayumi. They are different. If you are a ghostbuster/demon hunter/spiritually aware, you will be able to tell this is a hideously powerful and very old spirit! But that's really it. HOST BODY IS TOAST.
For the purposes of camp, rather than taking on a new body after being killed, she will just get back up and keep shanking, since I can't very well take over everyone's bodies in rapid succession. I will also tone back the death and bring more of the legend into it, which means you can get away with your life! Just tell me if you don't want to die, and all will be well.
edit: THIS IS PRETTY MUCH THE MOST FAMOUS URBAN LEGEND JAPAN HAS! Yes! You can know it! And of her! And if you have anymore questions this is the post for it!!