A faded Polaroid photograph. You have an ability to upload a personal family photo in the game, which will replace the "default" photo.
Will Mackenzie has this photo in his inventory and can refer to in times of need. When he looks at it, he is reminded of what he's fighting for, what he has to live for. This is manifested in the game by restoring a portion of his willpower.
But, with each use, the photograph fades a bit more, as though the memories that fuel it become less specific, less real.
The Willpower photo can be "recharged" in the game, by accomplishing certain goals, navigating certain choices, progressing through the story, etc. But always, you will have to use the Willpower photo very sparingly, because you never know what dangerous survival situation might be just over the next hill.