It always appears at the first angel stage of a chapter, however only in some chapters. If you close and open the app again, the effect disappears and you get a new master in the next angel stage. The master will never appear in chapters 11-13. Occasionally doesn't appear at all if you have a poor internet connection. In up-close dangers and flying bullets (events unlocked after completing chapter 3) the master will appear before the first boss at stage 9 of 20.