Thus!

"I have no friend, Margaret: when I am glowing with the enthusiasm of success, there will be none to participate my joy; if I am assailed by disappointment, no one will endeavour to sustain me in dejection. I shall commit my thoughts to paper, it is true; but that is a poor medium for the communication of feeling. I desire the company of a man who could sympathize with me; whose eyes would reply to mine" (Frankenstein, 4).

Aww.

Robert Walton, the author of the four letters, directly characterizes himself. He describes himself as "cool, preserving, and prudent," not humbly but rather candidly (7). That makes sense, as he's writing to his sister Margaret Saville, with whom he evidently has a healthy and trusting relationship. However, Walton's problem is that beyond his sister in London, he has no true friends; in the above quote, he directly characterizes himself as lonely by saying he desires "the company of a man" (4). He is also passionate and strong in will; "success shall crown my endeavors" (7).

What do you know? This exhausted European guy (almost reluctantly) comes aboard Walton's ship and reveals he has the same loneliness and the same passion as Walton. Actually, their characters coalesce perfectly. Walton calls the stranger -- let's call him, say, Victor -- "the brother of my heart" (11). He finds a friend, signalling that no matter how isolated someone may feel, he should never abandon the idea that his loneliness may be solved via a complete stranger who comes aboard his ship and feel just as isolated as he does. Metaphorically, of course.

Then, at the end of the fourth letter, Walton commences the frame story structure of the novel. Walton resolves to record "what he has related during the day," so the story shifts from Walton's point of view to Victor's point of view. We can infer that Victor's tale will be one of caution: "I imagine that you may deduce an apt moral from my tale; one that may direct you if you succeed in your undertaking, and console you in case of failure" (13).