How have the main characters in Mary Shelley's Frankenstein changed or grown throughout the story?
I would argue that the main characters -- Victor Frankenstein and the monster he creates -- don't really change very much at all. Victor never really becomes capable of accepting responsibility for the harm he causes to both his creature and to his loved ones. He doesn't seem to understand that his terrible neglect of his creature, as well as the way in which his creature was formed by him, are responsible for the damage that the creature has done. In the end, he tells Captain Walton, "During these last days I have been occupied in examining my past conduct; nor do I find it blameable." Though guilt seems to paralyze him at times, he doesn't confess his conduct in time to save Justine or Clerval; his own narcissism even seems to blind him to the fact that the monster intends to murder his bride, Elizabeth, instead of himself. He cannot accept responsibility for the creature initially -- he abandons it almost immediately after it gains consciousness -- and he cannot accept responsibility for it in the end.
The monster, on the other hand, began his life as a loving and benevolent creature, and though his behavior has changed throughout the text, his emotions seem not to have done so. He says to Walton, "Think ye that the groans of Clerval were music to my ears? My heart was fashioned to be susceptible of love and sympathy; and, when wrenched by misery to vice and hatred, it did not endure the violence of the change without torture, such as you cannot imagine." Ultimately, the creature is still good in his heart; he did not enjoy murdering the innocent and the lovely but felt he had no alternative to get the attention and cooperation of his maker. He says, "I was the slave, not the master of an impulse, which I detested, yet could not disobey." Therefore, ultimately, neither of these two main characters undergoes a fundamental change in the text.