Clarisse is indirectly characterized as pretty in "Fahrenheit 451". An example of this is when the author is describing her, "...her face was slender and and milk-white, and in it was a kind of gentle hunger that touched over everything with tireless curiosity." This is not directly expressed that she is pretty, but by the way she is described to the reader we can gather this information.
Clarisse is directly characterized as odd or different in "Fahrenheit 451". One example of this is when Montag says to her "You are an odd one". This is directly characterized that she is odd and different than a lot of other people in their society. The readers are directly told this about Clarisse.