The common understanding of Ethereals are they're typically people of great merit granted a form of "immortality" by the god Cievo.
In reality, they're essentially history stored in a separate realm called the Realm of the Composer.
While in the material plane, their bodies are "hard rhythm" that do not age, hunger, or become diseased, though they still have all the same biological functions as a living thing should they choose to pursue them.
This "promotion" occurs posthumously, typically moments after death.
Depending on the particular "legend" surrounding the ethereal, they may gain powers where they had none before (ex. Rose, the inventor of the Tuner, was a normal human, but gained Lightning abilities after becoming an ethereal.)
Typically Ethereals will stay in the mortal plane for awhile after their promotion (usually around 500-600 years, sometimes more, sometimes less) before returning to the Realm of the Composer.
Within the Realm of the Composer many other Ethereals exist. They prefer to stay here after awhile since they don't have to deal with the pain of being immortal and bonding with mortal that would most likely not become an ethereal themselves. The Realm of the Composer is a spiritual plane and while Ethereals can exist in the material plane, mortals cannot exist in the Realm of the Composer.
Depending on the ethereal they may choose to align themselves with modern day mortal maestros. They can interface with modern tuners just fine, but some (Such as the Noble Swords) opt to utilize Proto-Tuners even in the modern age. In these cases the Proto-Tuner will be a relic or artifact related to them. In combat, ethereals function like any other resonator, however their combat abilities typically surpass anything in the modern world, even going above the Grand Masters. However, these maestros tend to stray from the limelight and keep their relationships with ethereals on the down-low, as even if their resonator is an immortal god-like being, they themselves are not afforded the same luxury.
Ethereals can dismiss their hard-rhythm body and return to a spiritual form, being able to re-summon their body as needed. In their spiritual form, they cannot physically interact with the mortal plane, though if they have a maestro they can still communicate with them normally. Should they be bested in combat, they risk this rhythm body being destroyed and being forced into their spiritual form. The rhythm body can be recovered, but the recovery time is significant. So even though they are functionally immortal, they can be disrupted and without any intervention they can be rendered powerless to affect the mortal plane.
Well Known Ethereals
Seven Sages
Knights of Cievo
Noble Swords
Orchestra of Heaven
Rhythm Master, Noir
Tuner Inventor, Rose
Reartù and the Resonators of the Round Table
Ancestor of Language, Cunei