While I think it is possible to compare and contrast two characters using any of those techniques, the Venn diagram is likely to give you the best graphical illustration to compare two characters.
The question specifies a graphical illustration, so you should immediately throw out the outline. Outlines are incredibly functional at mapping out the main ideas of a paper or essay, but they don't function too well as a way to graphically brainstorm and organize.
While a pie graph can be incredibly informative, the purpose of a pie graph is usually to display numerical data in a way that is quickly and easily understood and compared. It doesn't make sense to compare characters with a pie graph.
Mind maps can be things of graphical beauty; however, a mind map's goal is to brainstorm and create various thought branch lines from a single word, concept, or idea.
A Venn diagram is the best tool listed to compare two things and look for overlapping similarities, and this is because a Venn diagram uses overlapping circles to show the relationships between two (or more) things. As an example, you would place each character above each circle. Then, for any personality traits that the character has, you would write them down in the circle. If a trait is shared by both characters, you would write that trait in the central area that is shared by both circles. The Venn diagram then makes it very easy to see what traits belong to both characters and what traits are unique to each character.