Dark Link
Dark Link is one of the most well-known enemy characters in the Zelda universe, mostly due to his likeness and ability matching that of the protagonist Link. It has been speculated that his popularity is so in part because he imitates almost every move Link makes, as if link is fighting himself rather than a seperate entity; that is, Link and Dark Link are opposite in intentions, but perfectly matched in ability (as well as action).
In The Adventure of Link, Dark Link appears under the title of Link's Shadow. He is the final boss in the game, and proves the most difficult opponent during Link's second quest to save Hyrule. He dodges most attacks Link attempts, but is vulnerable when he charges at Link.
Dark Link appears as a mini-boss in Ocarina of Time. He can be found in The Water Temple in a seemingly endless room. He mirrors most of Link's movements and attacks, usually defending himself in the process. To defeat him, Link must use the Megaton Hammer, or by using his swordand while L/Z targeting him do a horizontal slash on the oposite side of his shield ,which gives you an instant hit . Stocking up on magic potions and using Din's Fire is another option that makes the fight against Dark Link very easy. Once Link defeats his alter-ego, he will be able to continue on to a room containing the Longshot.
In Four Swords Adventures, Dark Link yet again makes an apearance; this time by the name Shadow Link. He proves to be quite persistant, and is featured as a mini-boss several times before he is finally felled.