From Hyrule Historia, the text reads:
When Link first set foot in the Twilight, the crest of the Triforce of Courage shined on the back of his hand; when he returned to human-form he wore clothes the hero wore. Link is taught mysteries from the ghost of the hero of time. Since he returned to his childhood, he’d felt regret that he would leave his name as hero. Therefore, the “son” of Link speaks proof of the courage he inheirited mysteriously.
Legacy of Courage - fanart inspired by the Hero’s Shade theory
Compared to other games in the Zelda series, Twilight Princess never offered a definitive backstory that relates it to other games. Although its timeline placement is firmly secured on the child arc, ages after Majora’s Mask, the game doesn’t begin with a set connection to another game. That is not to say that Twilight Princess is without a backstory entirely, because all throughout the game references are made to a hero of old, and the conflict surrounding him. Beyond mere references, in Twilight Princess Link comes face to face with the referenced ancient hero. In Twilight Princess, the Hero of Time makes an appearance, as the character that has come to be known as the Hero’s Shade.
“At last, the skills I have to teach you have entered the realm of
true secrecy. They are forgotten ways that do not leave our bloodline.”
Some Zelda Theorists believe that the Hero’s Shade is the spirit of the Hero of Time, Link from Ocarina of Time. Clickthrough picture to go to the Zeldawiki page and read basic theory points.