I heard Chapter 12 introduces a story about a ritualistic person. Can you tell me who this person was and what the chapter says about his visit to Shirdi?

Certainly. Chapter 12 sets the stage for a story about an orthodox Agnihotri (a keeper of the sacred fire) from Nashik, whose surname was Mule. The chapter explains that his arrival in Shirdi was not accidental but a result of the "strength of his past merits," noting that without such merit, no one can stay in Shirdi for even a moment. It's mentioned that his original intention for the visit was different from what ultimately occurred. However, a fortunate coincidence led him to gain the benefit of being in Shirdi and, significantly, brought his own Guru into his sight. This story is presented as a delightful illustration of the Guru's greatness.