Mary Shelley uses a multiple narrative in Frankenstein. A multiple narrative (sometimes referred to as an epistolary narrative) uses the voices of multiple characters within the text. Epistolary narratives can also use letters (such as Walton's four letters at the opening of the novel and the letters between the Frankenstein family) to detail parts of the plot and storyline. Some readers may forget, as they move through the novel, that Walton is responsible for telling the story of Victor and his creation.
The novel opens with four letters written by Walton to his sister, Mrs. Saville. The letters are written in first person (like most letters given their personal material). At the end of Letter IV, Walton agrees to hear the stranger's tale. The stranger declares that his destiny is determined, nothing can change it. Understanding the importance of the stranger's tale, Walton decides to take notes on the story.
I have resolved every night, when I am not imperatively occupied by my duties, to record, as nearly as possible in his own words.
Therefore, this sets the narrative voice--Walton's telling of Victor's story.
The narration becomes even more complex in chapters eleven through sixteen. These chapters belong to the Creature. That said, one must remember that the story is "given" to the reader through Walton's voice. Therefore, these chapters are three-fold in narration. While the reader understands the narrative voice to belong to the Creature, it is actually Walton telling Victor's recollection of the Creature's tale. Readers are actually hearing three voices at this point.
As chapter seventeen opens, Victor has reclaimed the narration (still provided using Walton's recording of Victor's tale). At the end of the novel, Walton reclaims the recognizable narrative, again through the letters to his sister (this is called a frame story--where the story opens and ends with the person telling the tale).