Ninja. Shinobi. Not much is known about them by civilians.
Here you wont find what is known about them by civilians.
Here you will find what is known by few people as this almost never happens and the knowledge of it has no practical application.
Shinobi depend on chakra more than one could imagine. The chakra changes a human being more than one could imagine.
The chakra determines what a shinobi is. The chakra controls every aspect of a shinobi's existence.
While not widely known chakra TRULY controls EVERY aspect of shinobi's existence. That is why missing ninja are rare. That is Why no shinobi village would accept a missing ninja into their ranks without a really good reason. The loyalty of shinobi also depends on their chakra. Not the sense of duty, not the love for their vilage makes shinobi fight till the last drop of blood for their home. It's their Chakra. it is not known why or how it is so. It is just a fact that most high-level and trusted shinobi know.
That is also the reason of existence of Root ANBU who hold no loyalty for Sandaime hokage, but whose loyalty lies within the village of Konoha.
Only one more thing is "widely known" about chackra. There are people with "special" chackras. Some are incredibly lucky, some can use elements to beyond what any other user of their chosen element can achieve. Sometimes there are whole families with special chackras that grant them the abilities known as Advanced Bloodlines or Kekkei Genkai. There are also "potential bloodlines" - people with "special" chackras that give them abilities unique only to them.
But there are some people with chakras that are truly special. Those chakras cannot be replicated and cannot become an advanced bloodline. This chakra influences everyone around the one who possesses it to an extent, actually changing the chakras of those around its wielder, always to the benefit of the possessor of the chakra. The magnitude of influence depends on many variables and no known patterns that would allow to understand who and in what way will be influenced exist. Anyone has a chance of having them, but VERY FEW POSSESS THEM. One such man created the art of chakra manipulation and it still exists almost unchanged to this day.
That man was known as Rikkudo Sennin.
Within Leaf village there were three shinobi known to posess this chakra to an extent. One was Senju Hashirama the first fire shadow. The second was Uchiha Madara. The third one was Namikaze Minato. Yondaime Hokage.
This cahkra is what originally was called the "Will of fire"
This is the story of the fourth posessor of such chakra in Leaf Village.