One NPC in Dark Souls 3 is Eygon Of Carim who turns hostile if the player attacks him. He is linked with another NPC named Irina Of Carim and if the player wants the former to be in the Firelink Shrine, the player must not kill the latter. Here is the guide showing how to unlock Eygon Of Carim.
Dark Souls 3 Guide: Eygon And Irina Of Carim Location
In the Undead Settlement, outside the tower, Eygon Of Carim is present. He can also be killed but keep in mind that he will turn hostile if attacked. At the Road Of Sacrifice behind the Stone Pillar near the hole, the player can summon him for an assist in the subsequent Boss Fight. He will also assist the player in the Dragonslayer Armor Fight.
Irina of Carim Location
This NPC can teach and sell Dark Miracles for the player making her a viable asset. She can be found near the Dilapidated Bridge bonfire in Undead Settlement inside a cell. Unlocking her and her abilities requires the player to get the keys from the Handmaiden for 1500 souls and the keys will only appear if the required ash is provided.
The player is rewarded by Irina with the Prayer Gesture, a rather useful commodity.