I think what you're talking about is a "foil" character. Foil characters need to have some significant things in common, but they differ in at least one important way. For example the characters may both be the same age, gender, class, etc but when presented with an important decision, one character chooses the "right" path and the other character chooses to do wrong. Or perhaps an author has two characters that are similar in some important ways and even act the same when presented with a certain conflict, but one character is male and the other female. The author then may be examining how the same situation affects characters of different genders by using foil characters.
Most often one of the characters is the novel's protagonist and the other a minor character. The minor character (as a foil) is present to highlight some major characteristic of the protagonist.
In the case of Frankenstein, perhaps the creature and creator are both outsiders but how they react to a public that dismisses and marginalizes them is different. This could be the author's way of highlighting the different reactions of society to modernity/science/technology/ or the "other."