Since the comparison is visual, I think it might be fascinating to compare two characters who look quite different where those differences also pinpoint their personalities.  For example, you can compare Atticus Finch and Bob Ewell.

A good place to approach the comparison is at the trial.  Atticus is very professional dressed and reserved.  He wears a coat, hat, and tie.

Atticus got up grinning but instead of walking to the witness stand, he opened his coat and hooked his thumbs in his vest, then he walked slowly across the room to the windows.

Bob Ewell, on the other hand, is rarely clean.  Besides being poor, he is unconcerned with his looks and does not often have a need for professional or even neat dress.  He lives a life unconnected from civilized society.

A shock of wispy new-washed hair stood up from his forehead; his nose was thin, pointed, and shiny; he had no chin to speak of- it seemed to be part of his crepey neck.

Scout notices that “scrubbed with lye soap in very hot water, his skin was white” meaning he does not clean himself often.