Hero's Spirit

The Hero's Spirit, also known as the Hero of Time's Spirit, Hero's Shade, Skeleton Warrior, or Golden Wolf, is an undead specter who appears in and teaches swordsmanship to Link. He is the Link from and  aka The Hero of Time, filled with regrets from being unable to pass on the "lessons of his life to those who came after him" after he was sent back in time to his childhood. In order to ease his regrets and finally pass on his knowledge, he teaches the seven Hidden Skills to his descendant, the Link of Twilight Princess.

The Golden Wolf
In Link's world, the Hero's Spirit takes the form of the Golden Wolf. The Golden Wolf is a ghostly-looking wolf, glowing a golden yellow color. Like the humanoid form of the Hero's Spirit, the Golden Wolf's left eye glows red while the right eye is missing. 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 and the Golden Wolf will appear as a symbol on Link's map. When Link confronts the Wolf, the Wolf will leap into Link, transporting him into an ancient arena near Hyrule Castle. The arena is described as being a different world and has an ethereal appearance. The wolf then finally transforms into ghostly humanoid form, the Hero's Spirit, who then teaches the young hero one of the seven hidden skills.

Appearance
In the other world, the Hero's Spirit's form has bluish skin and a pallid, decaying appearance, with a skeletal face and his left eye glows red while his right eye is missing. He carries a rusting sword and shield and wears ancient 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 humanoid form of the Hero's Spirit resembles the spirits in Hyrule Castle's graveyard.

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 Spirit 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.
 * As the Hero's Spirit is an incarnation of Link, this marks the first time in a game where Link has major dialogue that is not a result of the player's choice. This also marks the first interaction between two different Links.