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 despite the prince's attempts to stop him, and then seemingly dropped dead himself.

Identity
Although the true identity and nature of the magician is never revealed in the manual for The Adventure of Link, nor during the gameplay, Hyrule Historia has shed some light on his true nature. The book proposes two possiblities; due to his obsession with obtaining the Triforce, he is either a under the control or command of Ganon or possibly even an alter-ego of Ganon himself, much akin to Agahnim from A Link to the Past. Like Agahnim, he too acted as an advisor to the king before revealing his true intentions. It is also suggested that casting the spell on Zelda caused the magician to die because it spent all of his power at once.

Trivia

 * The magician bears an uncanny resemblance to Carock, the giant Wizzrobe boss of the Maze Island Palace.
 * As stated by Hyrule Historia, the magician also shares many similarities to Agahnim from A Link to the Past: both were close to the Royal Family of Hyrule but tricked them to further his own ends. Their pale blue skin and clawed hands are also extremely similar. The magician may have served as a prototype or concept for Agahnim.
 * 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.
 * According to Zelda Universe's official site for Zelda II: The Adventure of Link, the Zelda in the game is the same one as the one from the first game, and after casting a spell on her to make her sleep, the wizard fled to the Great Palace, which is why Link goes there at the start of the game.