Hero's Spirit

In Twilight Princess, the Hero's Spirit, also known as Hero's Shade, is an undead specter that taught Link each of the seven Hidden Skills throughout the game. The Hero's Spirit is also believed by some to be the spirit of Link from Ocarina of Time.

Appearance
The Hero's Spirit has bluish skin and a pallid, decaying appearance, with a skeletal face. He carries a rusting sword and shield which appear to be the Master Sword and Termina's Mirror Shield, respectively. He wears ancient armor which resembles the Magic Armor, which is covered with leaves in sections, and has details reminiscent of the Owl Statues. His sword's scabbard is belted to his waist. From the back, his armor appears to be filled with a ghostly mist, rather than flesh, and his torso is not connected to his legs.

The Golden Wolf
The Golden Wolf, also called the White Wolf, is the animal incarnation of the Hero's Spirit. The wolf is summoned by finding a Howling Stone, with the exception of the first time, in which he confronts Link directly. Once a howling stone is found, the song has to be howled in tune with the Spirit. He then tells Link to take "sword in hand" and find him, implying that Link had to return to human form before searching for the Wolf. Once Link encounters the Hero's Spirit, the Golden Wolf will appear as a symbol on Link's map. When he confronts the Wolf, Link is transported into an ancient arena. The wolf then finally transforms into his true form, the Hero's Spirit, who then teaches the young hero one of the seven hidden skills. The true form of the Hero's Spirit resembles the spirits in Hyrule Castle's graveyard.

Hero's Lineage according to Hyrule Historia
The book Hyrule Historia refers to the Hero's Spirit as the Spirit of Hero of Time,, and the Link from Twilight Princess as the Descendant of the Hero of Time, , confirming that the Hero's Shade is in fact the Hero of Time teaching skills to his descendant. His regrets are described as coming from being sent back in time to when he was a child and being separated from the timeline where he was a hero.

Trivia

 * The first song that Link howls in order to begin the quest of easing the Hero's Spirit's regrets is the Song of Healing.
 * Once Link learns the final Hidden Skill, the screen goes white and the Hero's Shade speaks his final sentence, "Go, and do not falter, my child!". This is similar to how the Seven Sages from Ocarina of Time speak their final words to Link.