who is the "ancient mariner" reffered to near the end of letter 2?

In Mary Shelley's Frankenstein, chapter Five, Victor Frankenstein quotes the lines: "Like one, that on a lonesome road / Doth walk in fear and dread / And, having once turned round, walks on / And turns no more his head / Because he knows a frightful fiend / Doth close behind him tread" (Penguin Popular Classic 1968 page 57, cited from Rime, 1817 edition). In the book's opening letters from Robert Walton to his sister, specifically Letter II, Walton explicitly mentions the poem by name and claims he "shall kill no albatross" on his journey.

He also reveals to his sister that ‘The Rime of the Ancient Mariner’ had always been a major source of his enthusiasm and his passion for the sailor’s life.

He promises he “shall kill no albatross,” a reference Coleridge’s “Rime of the Ancient Mariner,” in which the killing of an albatross has dire consequences. It is highly appropriate that this famous poem is one of the sources of Robert’s passion for the sea. One can even identify Robert with the ancient mariner, who started out like any other inexperienced sailor, but ended up a wiser man, due to his curiosity about mysterious things.