The Crestfallen Player is an NPC in Tiny Tina's Assault on Dragon Keep DLC
The Crestfallen Player is a reference to a character within the game, "Dark Souls". In the central hub are you cane encounter a fellow called the "Crestfallen Knight". He is a schemer so be wary of him. Still, listen to his story and offer Souls when you can.
The Crestfallen Player's appearance changed based on the status of the quest. When it is completed he will look like this: