Robert Walton is an explorer. On the particular voyage that forms the basis of the story's frame narrative, he is heading off to the icy wastes of the North Pole in search of uncharted lands and the knowledge they will bring.
Though a somewhat isolated individual, Walton isn't just in this for personal glory; he hopes that his expeditions will ultimately push back the frontiers of scientific knowledge, thus benefitting humankind as a whole.
This stands in stark contrast to Victor Frankenstein. While conducting his dangerous experiments, he seems himself almost like a god.
Frankenstein has such an unhealthy obsession with achieving his goal that he doesn't think through the consequences of his actions. It's only later on, after the creature comes to life and eventually goes on the rampage, that its reckless creator starts to realize just what he's done.
Even so, there are enough similarities between Walton and Frankenstein to make them bond immediately. Both are men of science, both are solitary men, and both find purpose in the process of scientific discovery. That is why Walton has embarked upon this treacherous, potentially perilous journey to the North Pole.
But unlike Frankenstein, he's not prepared to take crazy, irresponsible risks in order to fulfill his objectives. He knows when to let go. In this sense, Walton serves as a foil to Frankenstein, the very epitome of the “mad scientist” stereotype.