After all the battles you have to go through just to get hold of Ethel, she will unfortunately leave your party at some point in the story. Fortunately, there is a way to get her back!
How to get Ethel back in Xenoblade Chronicles 3 – Revive Ethel
Just like any other hero, getting Ethel to join your party involves completing her hero quest. Completing this quest takes time, and once you have Ethel in your party, you'll find that she's actually a very welcome addition.
Unfortunately, as you progress through the game's story, she will eventually leave your party. There's nothing you can do about it because that's how the story is meant to unfold. Fortunately, there is a way to get her back!
In this guide, I'll show you how to get Ethel back into your party.