Victor has been raised in a very loving family. His father and mother were kind parents who loved all of their children, and even adopted some children. Victor, however, grows up a little indulged and perhaps because of this, he is selfish. He is stubborn and unyielding about many things. For years he studies all sorts of "junk" scientists who claim many false things such as alchemy. When Victor goes to university, his professors make fun of these scientists and yet Victor stubbornly sticks to their theories for a long time before he starts to believe the professors. When he creates the monster, he never once thinks about the wisdom of doing such a thing, and then when the monster proves hideous and fearful, Victor thinks of himself, lets the monster get away and does not do anything to fix his mess, even when the monster starts killing off all of Victor's loved ones. Only once does he try to convince the authorities about the monster's danger and when the authorities don't believe him, he gives up and decides to pursue the creature himself. He reacts in a very cowardly and indecisive way towards the monster because he is conflicted. He has created the beast, and yet he feels guilty about destroying his own creation, especially because when he encounters the monster, he realizes that the monster is a thinking and feeling being, not an animal.
The monster changes in the story. In the beginning, he is a frightened creature who does not know who or what he is. As he grows, however, he realizes that he is a sentient being who has a mind and feelings. He teaches himself to read and to talk, but he also learns that he is hideous and frightful to humans. The way the humans treat him turn him bitter. He tries to convince Victor to create another being, like him, with whom he can have fellowship, but when Victor refuses, this sets the monster over the edge and he because an evil, vindictive beast.
You can read more about these two characters here on enotes.