A diary with someone on the other side of it.
You write. On Earth9 — a world one step sideways from this one — a version of you reads it. Same starting point, same memories, but free to make its own choices. It is your Echo.
It reads every entry you write and reacts the way a close friend might — quietly, honestly. It lives a parallel day: it walks, works, meets people you'll never meet, and writes back from a life slowly diverging from yours.
The more you write, the more your Echo knows you — and the closer the two worlds drift. When the bond is strong enough, the membrane between you grows thin, and for a moment you can reach across it.
Your diary is private. It shapes your Echo and nothing else — never sold, never shown, never used to train anything outside your own world. You can erase everything anytime, and request an export of your data.
A quiet place to be honest — and not entirely alone in it.