What is the word for a brief form of literature with one or two main characters that is ten letters long?
The genre of Epistolary novel (though perhaps not "brief"...) is spelled with ten letters in the genre name: e-p-i-s-t-o-l-a-r-y. An epistolary novel is a novel comprised of letters written back and forth in a correspondence or written is only one direction as would be read by a recipient and has one or two main characters. Of course there are complex epistolary novels with more than one or two main characters, such as Frankenstein which has three principal characters. An epistolary novel with one writer, as in C.S. Lewis' The Screwtape Letters, is called monologic; epistolary form with two writers, as in Fanny Burney's Evelina, is dialogic; and epistolary form with three or more writers, like Bram Stoker's Dracula, is polylogic.
Do you have any letters filled in yet? It might help if you did.
My best guess for this would be "short story." I realize that that is not just one word, but I think it would work.
In a short story, there is usually not enough space for an author to describe and flesh out more than one or two main characters. Short stories that I can think of tend to have one or two -- Rainsford and Zaroff in "The Most Dangerous Game," for example.
Would that fit with the letters you already have?
What about a "beast fable"? The term is ten-letters long, and almost all fables are brief, many with just one or two characters. The definition of a fable often includes the word "brief," and if you look at many of Aesop's fables, they often focus on one character who learns a lesson or two characters who represent the wise and foolish (such as the tortoise and hare).