check this out...
MSDN Platform SDK (Source)
Can I define my own character?
Yes. You can use any animation rendering package, provided that you can produce your images in a 2-bit (monochrome), 4-bit, or 8-bit color Windows® bitmap format. Then you can use the Microsoft Agent Character Editor, available for download at this site, to assemble and compile your animations.
copy and paste the above links in the browser, you will get list of links that
helps you create your own char.
Hope this helps.