The White Ninja Belt is a body item that you can earn in Card-Jitsu. It is the first belt any penguin can get. Once you have earned this belt, you are eligible to earn the Yellow Ninja Belt which allows you to progress through the game.
It is an extremely common body item due to it being the easiest belt to earn.
You only need one win to earn the White Belt, thus making it the easiest to earn.
When a player earns this belt, they also get a postcard from Sensei saying that you have started your journey. You will also get one for obtaining your Blue Belt, as well as one for obtaining a Black Belt.