As scholar Jeanne M. Britton points out, thus far, Walton has been the "letter-writer and first-person narrator." Walton accepts Victor onto his ship and feels an immediate sympathy for him, and as stated earlier, Walton seems to find a much-needed friendship in Victor. As Victor begins his story, the novel switches to his perspective. We then hear the Creature's story and within that narrative, we also hear Safie's story. Britton says, "The process by which the monster can identify with Safie and, in the act of transcribing her letters, adopt her voice marks the limit of the simultaneous experience of sympathy and shift in perspective that allows Walton to speak for Frankenstein and Frankenstein to speak for his creature. But by generating this transcription of letters, the desire for a sympathetic experience that seems within reach also produces a physical document that attests to both the truth of the monster's tale and the narrative and novelistic functions that sympathy performs." Walton's letters frame the whole novel, but Victor's narration also frames the Creature's story, which frames Safie's story. Each of these frames gives us a new story and gives us a complete picture of the characters in this novel. If the story were told from one point of view, there would be no truth to the story, as it would be completely biased. Walton is our unbiased framing device.
Britton, Jeanne M. “Novelistic Sympathy in Mary Shelley's ‘Frankenstein.’” Studies in Romanticism, vol. 48, no. 1, 2009, pp. 3–22. www.jstor.org/stable/25602177.