You need to defeat all bosses you can summon NPC companions for. These are typically the Beacon Bosses, alongside the Lightreaper Fight.

You also need to free her from her petrification in Forsaken Fen, as she is hidden near the first set of spear warriors in the entrance to the first village area. Look for some vines on the far left hand corner of the village, detectable by the sound of a woman singing. To free her, you Sanctify, a spell you get from the top of the tower entrance to Skyrest Bridge, which you can access after defeating Pieta. You will need to buy a Radiant Catalyst from Exactor Dunmire if you did not start with one.

If you fail to free her before the Hushed Knight beacon boss, you will auto fail the quest. Also, make sure to farm Vigor, and pay her off, as failing to do so in an adequate manor by Upper Calrath will result in her turning hostile and ganking you as you near the end game.