Wait a second, if the reason some characters can hear us and others can't is DETERMINATION, why can echo flowers repeat us? Do echo flowers have DETERMINATION? If so, why and how? Or is it just a different situation with echo flowers than characters?
Some characters can only hear you is because of 2 major elements- they have determination, and they are going through/had gone through the underground, but the reason why the echo flowers can repeat you is because you are just voice (mostly) to these character. Echo flowers can capture and play back voice messages. ( as is what happens in the game)
but echo flowers do not have determination, it is just their magical properties