Secret Ninja

To become a ninja, you must challenge others by facing them on a 1 on 1 challenge, your last match against Sensei will determine if you are a ninja or not.

The most important part of Card-Jistu is, Fire beats Snow, Water beats Fire, Snow beats Water, these symbols will help you succeed in your challenge. 

There are over 9 belts that you could win by challenging others on a 1 on 1 battle.

Here is a small clip provided by a Club Penguin User on how to become a ninja: