Greirat the thief is a new NPC you can interact with in Dark Souls III. To free him from his cell at the High Wall of Lothric, first you need his cell key. After getting past the large tentacle hollow on the roof, take the ladder down and enter the room with the Lothric knight. Just past him is a large house where you’ll find the cell key lying at the bottom of some steps on the ground floor.
To get to Greirat’s cell, head to the Tower on the Wall bonfire and take the ladders all the way down. You’ll come to a cell with Greirat sitting inside. Agree to fulfil his promise and he will go to Firelink Shrine. Next, move ahead to the Undead Settlement and grab Loretta’s Bone by cutting loose the body on the balcony of the very first house. Bring the Bone back to Greirat and tell him of Loretta’s fate. He’ll let you keep the Blue Tearstone Ring.