What kind of characterization can you do for a character in fictional setting when you are given just a one-line description?

Give a paragraph or more of characterization of the one-line character given above, including some of the stuff like their background, habits, personality and whatever feels right for you to make that character alive. Then make the next poster do just the same by giving a one-line description of another character!

Here it goes.

Mark, the psychopath with a heart