A foil character is one that contrasts with another character, usually the protagonist. The contrast is used to emphasize certain traits of the protagonist.
Although the description of Victor and the Monster can vary, Victor is usually described as selfish and arrogant, and suffering from a god-complex. These characteristics are revealed through Victor's actions in the first part of the novel. His total disregard for his friends and family and his overly-ambitious pursuit of knowledge portray him as a man willing to go to any extreme to reach his goals. In addition, Victor abandoning the creature shows him as a weak man unable to accept the consequences of his actions.
However, the Monster is often considered compassionate and selfless. As the Monster reveals his tale to Victor, the reader is shown how human the monster is. The Monster takes refuge in the small things in life--fire, reading, family. In addition, the Monster recounts the "barbarity of man". Upon the Monster's first visit to a village, he is attacked and rejected.
The treatment of the monster is the complete opposite of Victor, making the monster a foil to Victor. The monster is portrayed as grotesque and abnormal while his thoughts and feelings show true compassion. But, he is still rejected by Victor and society. Victor, on the other hand, is accepted by everyone even though his actions appear to be truly grotesque.
Another aspect of the monster being a foil to Victor is the plot. In a framed narration, such as Mary Shelley's Frankenstein, there are several plots taking place. Sometimes the plot of one becomes a foil to another. In the case of Frankenstein, the monster's plot becomes the foil to Victor's. Victor's tale is presented at the beginning of the novel, beginning with his privileged childhood. The monster's tale is presented in the second half of the novel, and, it too, begins with the monster's childhood, however, clearly not a privileged one. The differences in the two character's "lives" allow the reader to gain perspective into each character's actions and decisions. It is used to emphasize, in this case, the extreme consequences of Victor's behavior.