Ninjas are required for you to play the game. In order to get ninjas you have to recruit them and each have certain skills and attributes that makes them unique and great.
For more information about recruiting, go to Tavern and Ninja Club.
In total there are 65 Ninjas that you can recruit… with some ninjas that are special such as Hinata and Cursed Seal Sasuke. Refer to the Tavern and Ninja Club for the recruitable ninjas.