Moonscars

In the Depths you will eventually uncover the key allowing you to move west, over a spinning cog blocking your path.

You will discover the Great Mills mirror.

You will die and be warped back to the Mould Workshop when interacting with said mirror for the first time, just as with every other Doppelganger up to this point. When you warp back to the Great Mills mirror in the Depths, you will find yourself in diaglog with the Kind doppel. When prompted you will hold A to spare her. Then move on with the game.

Note: When sparing the Doppel you will lose your Special Attack. Rather than wait to kill Wanda before you get it back, just go back to the mirror and buy a Weapon Rite with 3 glands.

When you reach the end game in the Apex, you will encounter the Kind doppel on a platform just before the Red Irma fight...(the point of no return). Speak to her and exhaust her dialogue to pop the acheivement.

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
You can abuse cloud saves to get both this achievement and the opposite one for:

Once you activate the mirror that kills you, go to the options menu and exit to title. Then Guide > Quit the game. This should prompt a cloud save upload.

Wait a minute.

Loadup your save file, kill the Kind doppel, then Guide > Manage game (don't quit!) > Delete local save ONLY (not cloud save!). Then load up the game, it should sync with your cloud save, and have you back at the mirror able to spare the Kind doppel and continue with the story.

-it's worth noting that if this doesn't work for you or you don't want to try it at all..you WILL need to do another play through. The most efficient approach to doing 2 playthroughs is to prioritise sparing on the 1st playthrough rather than killing, because to get to this point only takes a couple of hours, and the achievement for killing pops straight away rather than having to complete 90% of the game before you get it.