The Crestfallen Warrior is a nameless NPC who resides at Firelink Shrine. For many players, he is their first point of contact in Lordran. He offers tips and advice to the player character in their quest, seeming to have a general knowledge of how the Chosen Undead should proceed, though it is never revealed how he obtained this knowledge. It is likely he was an Undead who came to Lordran to break the Undead Curse but became discouraged at how difficult it was, so now resides at Firelink Shrine waiting to go hollow.
Once the Chosen Undead rings the second Bell of Awakening and therefore awakens Kingseeker Frampt, the Crestfallen Warrior complains about the noise, the snores, the lethal breath and the stench of the primordial serpent. He then states that it may be time he did something about it. You will next find him in the New Londo Ruins where has has gone hollow and will attack you on sight.