Robert Walton is a very aspirational character. This is more prominently seen when he remembers his younger years. He tells that he has been self-educated in naval information before he became a seaman. Also, during his time on the ship, he notes that when he wasn't involved in crew work, Walton studies several branches of study in order to gain the "greatest advantage".
The fact that he aspires to know so much about several subjects emphasizes the size of Victor Frankenstein's high aspirations. Walton is not only useful as a framing devise, but one could understand more clearly the mindset of Victor with what Walton writes to his sister. In fact, Walton and Victor are very similar in family life. Walton is far away from his relatives and engrossed in his educational goals; Victor is far from home and obsessed with his studies and human creation. Walton has a close relationship with his sister; Victor is very close to Elizabeth, who is much like a sister (aside from the dream in which Victor almost kisses her). This is revealed with the interactions between the two.
Through his interaction with Victor, Walton's wish for a friend seems to come true. They have a lot in common, but there seems to be restlessness in Victor that suggests that there is something very troublesome in his life. This passion is especially prominent when Victor shows anguish during a conversation with Walton. Such intense emotion emphasizes how large the issue is to Victor. He can't even hold back when he is in pleasant company.
