Appeal is one of the animation principles, which makes the character or design stand out. Viewer feels realism and it generates further interest when appeal is added to the design. Character should be capable of creating a pull when presented.
They should have charisma.
Appeal is not only used for the lead characters. But also villian, sidekick and every other character.
Instead of using the same variety of shapes to all the characters, You can play with them in shape variation and use it according to their personalities. Every good character have clear shape.
Playing with proportion and Pushing it
Exaggerate the things which looks interesting in design. Push the shape further.