I heard Chapter 12 introduces a story. Who is it about and why is it being told?

Yes, the latter part of Chapter 12 introduces a story that is described as 'very delightful' and illustrative of the Guru's greatness. The story is about a ritualistic and orthodox Agnihotri, a keeper of the sacred fire, from the holy city of Nashik. His surname was Mule. According to the text, he came to Shirdi due to the strength of his past merits, as it is said that no one can stay in Shirdi even for a moment without such merit. The purpose of narrating this story, as stated in Chapter 12, is to demonstrate the greatness of the Guru and to indicate the love that a Guru has for his devotees.