Magician

The Magician is the character who, accordingly to the manual of The Adventure of Link, was close to the King who had ruled Hyrule and maintained peace with the Triforce. After the King died, his son (the Prince), who had been looking all over for the Triforce, was told by The Magician that the King told Princess Zelda, the Prince's sister, about the Triforce but she refused to tell him when he questioned her. The Magician then angrily threatened to place her in an eternal sleep if she didn't tell him. After she refused The Magician, he used his remaining life force and placed a spell on her and immediately died. No more was ever said about The Magician even after having such an important role, leaving many possibilities regarding his true identity.

Possibilities
The Magician bears an uncanny resemblance to Carock, the boss of the Maze Island Palace. One would have to ask why the Magician would kill himself or at least knowingly risk his life to punish Princess Zelda if she didn't tell him about the Triforce. There would be no gain for him as he would have died for nothing. It's possible that the Magician while casting his spell broke through Zelda's mind, learned the whereabouts of the Triforce of Courage, and at that point became Link's Shadow (Dark Link). It may not be a coincidence that the Shadow is the boss that Link has to fight to get the Triforce of Courage.

It's also possible that the Magician is similar to Agahnim of A Link to the Past and is merely an alter-ego of Ganon created prior to his battle with Link in The Legend of Zelda as insurance in case he were to fall. This would mean that just as he used his Agahnim identity to overtly open the seal to the Golden Land and overthrow the King in A Link to the Past, he would have covertly used the Magician to get close to the King as told by the manual of The Adventure of Link. A spinoff of this theory could be that Ganon created the Magician as an alter ego who died when casting the spell on Princess Zelda and subsequently became Link's Shadow (Dark Link). It would be fitting that Link's opposite, Ganon, would become his shadow, or dark side. Or in other words that Ganon and the first ever Link throughout time (one we may have not yet seen) were born/created at the same time as opposite sides of a coin. This would mean that there must always be a Link and a form of Ganon. Ganon's form in The Adventure of Link is seen to be of a shadow at the Game Over screen and again as a shadow on the map screen as Link is absorbed into the areas of the map.

Trivia

 * The monster presiding over the wedding ceremony in the "A Hitch in the Works" episode of The Legend of Zelda TV series looks similar to the Magician.
 * The Magician shares many similarity to Agahnim from A Link to the Past: both were close to the Royal Family of Hyrule but tricked them in order to serve Ganon. The Magician may have served as a prototype or concept for Agahnim.