I hope we can discard concerns about “alts” and “mains”.
Before the game will begin awarding any character Echoes of Ny’alotha, you want to first log into a character (or characters) who have Rank 3 Essences. That’s it.
Once you’ve logged into a character who has a Rank 3 Essence, all of your other characters (that are eligible) will begin seeing Echoes as rewards.
Thus, if you have two characters who have different Rank 3 Essences, you’ll want to log into both of them first, each unlocking the Echoes for the other.