This "man of immemorial age" (11.12) is only around for a few pages in the letters of Ermes Marana, but you can tell right away he's high up on the Symbolic Importance Scale. Here's what we know about him: he's an "old Indian" who lives in South America and who, "according to some, is the universal source of narrative material, the primordial magma from which the individual manifestations of each writer develop" (11.12).
So yeah, he's kind of a Big Deal.
This guy fits into the story's larger commentary on storytelling by suggesting that one single source could be responsible for all of the stories in the world. Kind of gives a sense of order to the network of confused authors and titles of books, right?
But don't get too excited. This figure has only a mythical status in the book, and the story seems to discard him just as quickly as it picks him up.