Focus on a character trait. This can be a physical trait or an aspect of personality. The trait should help define or mark the character.
Example from Literature
His ignorance was as remarkable as his knowledge. Of contemporary literature, philosophy and politics he appeared to know next to nothing. Upon my quoting Thomas Carlyle, he inquired in the naivest way who he might be and what he had done. My surprise reached a climax, however, when I found incidentally that he was ignorant of the Copernican Theory and of the composition of the Solar System. That any civilized human being in this nineteenth century should not be aware that the earth travelled round the sun appeared to be to me such an extraordinary fact that I could hardly realize it.
“You appear to be astonished,” he said, smiling at my expression of surprise. “Now that I do know it I shall do my best to forget it.”— Sir Arthur Conan Doyle, A Study in Scarlet