Asymmetry.  Diversity begins with how asymmetrical your character appears; imperfections that make your figure more realistic when an observer sees them for the first time.  A slight hunch of the back, a limb shorter than the other, a shoulder droop, a broken nose, etc. will give credibility to a character's presentation and attitude in an acceptable way.

Also, you may want to exaggerate emotional expressions, especially when telling a story through comic-style illustrations.  Use as many facial morphs to convey the emotional state of your character