What it takes to be a ninja?

Single Minded Perseverance

    It can be answered in three ways:
1. By using meanings
2. By using history
3. By way of skills and philosophy of ninjutsu

   To answer this by the way of using meanings, we must first understand 'Who is a ninja?'. We must also understand 'What's ninjutsu?'. These two questions can not be easily answered and hence we must find the individual meanings of each of the two words, 'ninja' and 'ninjutsu'. This has been discussed in detail in an earlier post Introduction: Ninja and Ninjutsu. After reading this post, you can infer that there are three requirements to be a ninja: heart under the blade; hiding; perseverance. By following these three basic rules, you are following the shinobi way.

   To understand 'by using history', I mean to refer to the historical documents which includes manuals, densho and other original scripts, to find the meanings to this question.
   Iga Traditions say, "When you make something a deep secret, it is called Shinobi".
   Koka Traditions say, "The ideogram for Shinobi comes from ideogram for patience. Thus it is called Shinobi because these are skills to endure any hardship or difficulties".
   Togakure Tradition says, "It is true that the ideogram of 'nin' or shinobi means concealment; It is also true that it means patience and endurance".
   Kumogakure Tradition says, "The image of a shinobi is like that of a shadow appearing after the moon is covered with clouds".
   What you should keep secret is your name, plans, skills and affiliations. This is the way of shinobi and you must follow it to be called a true ninja.

   By using skills, it doesn't mean that you show off stunts in public, it means that people don't know how skilled you are, actually as a shinobi. No matter how good you are at performing several feats, you are only a medium-level ninja if your skills are known. Sun Tzu says, "Those skilled in defense conceal themselves in the lowest depths of the Earth. Those skilled in attack move in the highest reaches of the Heavens. Winning battles such that the whole world cries 'Excellent!' is not the highest excellence". A skillful shinobi has his heart under the blade and thus do not cry out loud on his success, rather he stays silent as if nothing has ever happened. Further that you work with endurance and patience towards learning more skills. This is the way of Shinobi. Furthermore, for practical skills you must have learned from the authentic masters of Shinobi. You can not claim to be a ninja having inherited traditional skills of ninjutsu without a proper lineage. Ninjutsu is no martial art that you can learn in a dojo, it's something transferred directly from the heart or spirit of the master to that of the disciple.
   The philosophy of ninjutsu is the same. If your inner is not at peace with your body, if you are impatient and if you can not stand a victory or loss, then you are not of worth to be called a shinobi, because you are simply giving away your position, which means that you can no longer stay hidden, which means that you have lost the purpose of a shinobi. To be a shinobi it takes three things, as Hatsumi Soké would say: Peace, Patience, Perseverance, and my own belief is that you need three things: Patience, Endurance and Following the Shinobi Way. In my view, following the shinobi way means that you apply the principles of ninjutsu in all matters of your life.