I really like how you implemented the echo mechanic. I found that the early levels taught me alot about how the mechanic worked, making progression feel natural. The shader used for the echo effect is rather nice. The pace of introducing additional environment features was spot on too.
Choice of colour schemes is great. As simple as it is, the yellow background actually gives the game a very distinct and pleasent feel. This combined with the music really sells the aesthetic of the game for me.
For improvements, I did not like the keybind of Shift/Ctrl (although I found using Right CTRL ok). Maybe set it to [E] by default?
High quality and interesting platformer. Well done!