I am a Ninja!!! Well, I become one awhile ago on Friday the 19th December 2008.
To beat Sensei on black Belt, all you have to do is play against him 10 times. on the 11th game you will beat him and he will award you the Ninja Mask.
Once you become a Ninja, you will have access to the Flying Emporium. It’s located here:
In the Martial Arts Catalogue, you can buy a Ninja Suit, a Dojo Igloo, a Rice Paper Wall Screen, a Stone Lantern and a Hand Gong.
If you are a Member and Buy a Ninja Suit, you can become invisible by pressing Dance.