Spirit of the Hero

The Spirit of the Hero is a force referenced within the Legend of Zelda series. Little is known about the exact nature of it but appears to be the essence of what makes each protagonist in the series Link. Presumably it is present in all games featuring Link, however it has only been directly referenced on several occasions.

The Legend of Zelda: Twilight Princess
After Link recovers from his Wolf form by collecting the Tears of Light, he meets the Light Spirit Faron. Faron tells Link that he is connected to the ancient hero chosen by the gods and that a portion of that hero's power resides within him.

The Legend of Zelda: Skyward Sword]]
Upon his defeat, Demise explicitly refers to the Spirit of the Hero. He curses the Spirit of the Hero so that an incarnation of his hatred would constantly stalk whoever has the Spirit of the Hero and force them to live a life of conflict.