then what exactly is the objection here?
In practical terms you are asking for a semi-solid, grayscale, Tupac Shakur hologram, that used to be at the Coachella Festival, be ruled as
Hidden, despite the fact the hologram is standing right in front of it’s target, stabbing with a rapier.
A clearly visible drone in Afghanistan, should not count as
Hidden, just because you cannot see the operator in another part of the world.
Even if the operator is a halfling, doesn’t change the visible fact that the point of delivery of the attack is
quite visible.
Now if both the Echo and the Halfling Echo Knight were Hidden, (say the Echo was in an area of the Darkness spell, and the halfling was Hidden, outside the Darkness effect), my ruling would be the attack gets Advantage.
By RAW an 7th level Echo Knight of
ANY race, under the effect of an Greater Invisibility spell, would have advantage on any attack that used the Echo Avatar as the point of delivery....even if the completely
visible Echo was 1000 feet away.
strains credulity
Back to the original post, the Assassin subclass could work well with Echo Knight.
Careful use of the Echo means one can use the Echo and Unleash Incarnation to get attacks on Surprised creatures, and creatures that have not gone yet in combat, (and thus get Advantage).
Alas, you would not get Death Strike, but Assassinate and Unleash Incarnation, would be scary and effective.