In Mary Shelley's Frankenstein, Walton reveals much about his character. While this is not a direct characterization (where the author provides all characteristics about the character directly) provided by Shelley through Walton's voice, readers can tell much about Walton based upon his one-sided dialogue (the letters) to his sister (which is indirect characterization-the assumptions or decisions made about a character based upon information inferred).
First, Walton is a man who truly loves his sister. One could assume that if he did not, he would not waste valuable expedition time trying to correspond with her (given the reader never actually knows if the letters make it to her.
Second, readers come to understand that Walton is a man driven by a dream.
This expedition has been the favourite dream of my early years. I have read with ardour the accounts of the various voyages which have been made in the prospect of arriving at the North Pacific Ocean through the seas which surround the pole.
Third, readers come to understand that Walton will not be deterred from his desire to discover the seat of magnetism.
Walton understands the outcome of his expedition may not end well.
If I succeed, many, many months, perhaps years, will pass before you and I may meet. If I fail, you will see me again soon, or never.
It is here that readers come to understand that Walton will seek out the pole regardless if it ends in his own death.
Lastly, readers come to understand that Walton is a man who needs others. In Letter II, Walton admits to his sister that he is friendless.
But I have one want which I have never yet been able to satisfy; and the absence of the object of which I now feel as a most severe evil. 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 sympathise with me; whose eyes would reply to mine. You may deem me romantic, my dear sister, but I bitterly feel the want of a friend.
Above all else, Walton desires friendship.