Help Keira decide on a very important decision for herself and for Geralt.
The Witcher 3: For the Advancement of Learning
Once you've completed A Favour for a Friend this quest will begin.
You need to get over to Fyke Isle and head to the marker where you'll find Keira.
She has a decision to make with your help and it's really crucial to how the rest of the game will play out. If you agree to her suicidal plan you won't see her again in the game, and if you don't let her take the letters you'll have to fight her.
Instead, talk her out of the plan and convince her to go to Kaer Morhen. This will ensure Keira will be available to aid the witchers against the Wild Hunt later in the game.
The quest is now complete.