What is the difference between Frankenstein's inner self and his outward appearance?
The description of the creature is horrifying. He is yellow-skinned, his eyes are a dull yellow, and he is stitched together from the parts of dead people--most likely criminals. As a result, he appears to be scarred across much of his body, and the skin barely covers the mass of muscle and arteries that run underneath it. His lips are straight and black, and his hair is a "lustrous" black. In contrast, his teeth are "pearly" white, and he stands eight feet tall and is "proportionably large". He is an outcast from the very beginning due to his abnormal height, but his garrish appearance makes him even more abominable. The white teeth and lustrous black hair which would be considered beautiful make him even more ugly as they are in stark contrast to the rest of his features. Chapters four and five of the text hold these jewels.
In chapters 11-14 we get the creature's point of view. We know from his speech that he was born benevolent and kind. He smiled at his creator/father and hoped to detain him. He attempts to connect with humans several times, only to be beaten, burned, chased, and abused. He says he is full of love--rescues people in distress, helps the blind and the poor, learns about humanity, love, hate, war. It is only when he discovers the nature of his creation and his abandonment that he seeks revenge. His outside doesn't change, but his inner nature is effected.
In addition to the above answer, the monster is the combination of many different humans - but he has his own unique personality. He has clear rationale for his plan and what he intends to do. He has specific goals just for himself. He does not see himself as a "mixed" being - he sees himself as an independent and sovereign being. As an individual, he goes through the life process of a human - as a "newborn", he deals with the sensations of his body:
and it was, indeed, a long time before I learned to distinguish between the operations of my various senses.
He learns speech, like a toddler:
I discovered the names that were given to some of the most familiar objects of discourse; I learned and applied the words,
He learns emotions, and sharing emotions with others:
when they were unhappy, I felt depressed; when they rejoiced, I sympathized in their joys
And he comes to the point where he begins to think critically:
Of what a strange nature is knowledge! It clings to the mind when it has once seized on it like a lichen on the rock.
Shelley strongly suggests, through the contrast between inner and outer, that the soul is separate from the vessel that carries it through the world.
The creature is made of scavenged body parts, chemicals and brought to life with a spark, on the outside, he is hideous, deformed and frightening. However, on the inside, the creature develops sensitivity and understanding of human relationships as he observes life from a distance.
The creature expresses a wide variety of human emotions, including love, friendship, anger, revenge and the longing for a companion who looks like him.
At times, the creature resembles a wildly confused teenager in the midst of rebellion, feeling rejected by society and out of place with his own family, Victor Frankenstein, he searches for meaning and purpose in the world. He is frightening and unable to fully control his reactions to stimuli in the environment. The comparisons are very real.