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 became king and inherited everything he owned, except for the Triforce. 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 cast a spell on her and immediately died.

Identity
It is 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.

Trivia

 * It is unknown why the Magician killed himself or at least knowingly risked his life to punish Princess Zelda when she didn't tell him about the Triforce
 * The Magician bears an uncanny resemblance to Carock, the boss of the Maze Island Palace.
 * The monster presiding over the wedding ceremony in the "Hitch in the Works" episode of The Legend of Zelda TV series looks similar to the Magician.
 * The Magician shares many similarities to Agahnim from A Link to the Past: both were close to the Royal Family of Hyrule but tricked them in order to get to the Triforce. The Magician may have served as a prototype or concept for Agahnim.