Greirat of the Undead Settlement

Greirat of the Undead Settlement is a character and merchant in Dark Souls III.

Location
Found in the High Wall of Lothric. Greirat is first met while imprisoned in a cell below the Tower on the Wall bonfire.

From the bonfire, head downstairs into the room with the Lothric Knight and an Assassin. Go downstairs once more and then down the ladder. Move through three rooms and then downstairs and to the right, Greirat's cell is at the bottom.

The Cell Key is required to open the cell; it can be found in the room with exploding barrels and several Large Hollow Soldiers and Starved Hounds, below the roof with the Pus of Man. From this roof, head down the ladder on the right, turn around and go inside the building. Move through this area, down two sets of stairs until reaching a large room that has an Estus Shard lying on an anvil. The Cell Key is in a corner of this room, down a small staircase guarded by a Starved Hound.

After being freed, Greirat will relocate to Firelink Shrine and stay near the base of the staircase on the west wing.

Questline
After thanking the Ashen One for freeing him, Greirat asks a favor; he bestows the Blue Tearstone Ring to the player under the condition that they find and give it to a woman named Loretta who lives somewhere in the Undead Settlement.

Loretta's Bone is found on a body hanging from the balcony of the first three story building encountered in the Undead Settlement. It can be struck down by attacking it with a weapon or thrown item. Once the player collects the item and returns to Greirat in Firelink Shrine, he allows them to keep the ring and becomes a merchant. After this, Greirat's services will become temporarily unavailable as he grieves for Loretta. Interacting with him at this moment will bestow the "Curl up" gesture. The player needs to re-load the area in order for him to return to normal.

Since he is a thief, Greirat will occasionally ask the player if they would like him to go pillage items from various areas. Agreeing to this will make him leave the shrine until an area boss (any) is defeated. Upon his return, his stock of items will be upgraded.

Once the player reaches Irithyll of the Boreal Valley and returns to Firelink Shrine to speak with Greirat, he will again ask the player if they would like him to go and pillage Irithyll. He will die in Irithyll unless certain conditions are met:

During the player's travels through Irithyll, the player must not activate the bonfire at Irithyll Dungeon. At this point, the player has two options they can choose from to save Greirat:
 * 1) The player must have completed Siegward's questline up to the point where he can be encountered in Irithyll. His dialogue should not be exhausted, as this will trigger his appearance in the Profaned Capital and he will not be in Irithyll to save Greirat.
 * 2) The player can tell Patches about Greirat's location, provided they did not purchase the Catarina Set, as Patches uses the armor to rescue him.

If Greirat dies during this part of his questline, the player can retrieve his ashes from his body located in the far-right corner of the left path in the sewers of Irithyll, surrounded by three Sewer Centipedes. Bring the ashes to the Shrine Handmaid and she will update her stock of wares with those Greirat initially sold.

Once the player reaches Lothric Castle and returns to speak with Greirat at Firelink Shrine, he will ask the player if they want him to pillage the castle. There is no way of saving him if he is sent to pillage Lothric Castle; if the player wishes to keep him alive, they must refrain from sending him when he asks for permission to go. However, sending him there completes his questline. Upon finding his ashes, they can be brought to the Shrine Handmaid and new items will be added to her stock in addition to those previously sold by Greirat.

Greirat's Ashes can be found on the rooftops of the Grand Archives, past the first Gargoyle the player encounters. Continue along the rooftop outcrop where below there is a Corvian Storyteller and a group of Corvians. There will be a small break in the ledge where the player can drop down to the lower level without alerting any of the enemies. Continue forward where there will be an adjacent rooftop, jump to it to  find Greirat's ashes as well as three Titanite Scales.

Trivia

 * It is never explained what kind of relationship Greirat had with Loretta, but due to her age, gender and place of residence, she could have been a relative or some sort of motherly figure.
 * Though Greirat wears the Thrall Hood, he has a human character model, like the player, and it is not part of his model like the Thrall enemies.