Here are some great resources that will help you refine and hone your ninja character design skills.
The Character Design Blog
Weekly interviews with leading character designers, including links to their websites and galleries of their work.
Character Design References
A facebook page with links to artists and great character design resources
Character Design Challenge
An open facebook group. Jump in and join the weekly challenge, or just browse the amazing submissions
Character Design Resources on Pinterest
An incredible collection of tutorials, tips, reference material and work.
The Twelve Principles of Animation
These principles are also very useful for illustrators to follow in order to create dynamic action and narrative tension in their images