Japanese
ninjas are Japanese
That is where ninjas originated. But they are actually known as Ninjanese. Some people think that Ninjas are Chinese or Japanese, but that is not the correct orrigination. Ninjanese is in fact the correct one.
Ninjas were Japanese assassins and yes of course.
There is no such thing as a Chinese ninja. Ninjas originated from japan. however China had far superior karate masters. So basicaly the Chinese were highly skilled in karate but not with weapons. whereas the Japanese knew karate but they weren't as good as the Chinese but the Japanese were highly skilled with weapons. so in my opinion, in a fight between a feudal Chinese warrior and a feudal Japanese warrior (ninja) , the Japanese ninja would win
They are Japanize
Ninjas originate from Japan from their skills that are told in ancient Japanese history
Ninjas are a feature of Japanese history and did not feature in any other culture.
A vagabond is like a Chinese ninja. Ninjas are Japanese. I don't know much about either, but I don't think that either of them exist anymore.
Whoever hired them.
It depends on your perspective. Ninjas were a secretive class of Japanese agents trained in the arts of espionage and assassination -- the style of stealth and martial arts used by ninjas is called ninjitsu.
Samurias fought ninjas. Samurias were part of the Japanese dynasties, and ninjas fought them. It was pretty much for power in the dynasty along with more land.
Chinese has different sounds, lettering, and meanings to their writings. Japanese is the same way but Japanese do borrow the Chinese lettering from the Chinese and but the Japanese do have their own pronunciation for it. They are still different languages.