Anyone can learn to become a Shinobi, much like a Paladin, and harnass the powers of Ninjutsu, so long as those with the power and knowledge are willing to teach the individual in question. Doma is no where as near as xenophobic as Ishgardians, and spoilers aside, the lore lends itself to allow for anyone to become a Ninja if they so choose.
Obviously, to state the obvious, a good (poor if you are feeling lazy) story is needed to explain how a character is able to manipulate their chi (aether) to perform Ninjutsu and "release" the powers of man, heaven, and earth.
Also, nothing is saying a player's echo power can't be completely fluff. Golden Rule: do what is fun.
If you don't accept another players concept, fair enough, but players should play whatever best suits their time and gives them the greatest happiness.
Obviously, to state the obvious, a good (poor if you are feeling lazy) story is needed to explain how a character is able to manipulate their chi (aether) to perform Ninjutsu and "release" the powers of man, heaven, and earth.
Also, nothing is saying a player's echo power can't be completely fluff. Golden Rule: do what is fun.
If you don't accept another players concept, fair enough, but players should play whatever best suits their time and gives them the greatest happiness.