Dark Link is a recurring boss in the series and is a doppelgänger of Link. Also known to zelda as a twisted echo in four swords manga. Dark Link is usually solid black with red eyes but is white with purple hair in the Four Swords manga plus. In general, Dark Link just copies Link's swordplay, but in some games he is able to use Link's full arsenal of weapons. He is formed in various ways between the games, though always involving some sort of magical summoning. When he first appears in The Adventure of Link, he is the final boss of the game, and was created by a mysterious wizard as a test for the Triforce of Courage.
In 2010 IGN ranked Dark Link as 77th in Top 100 Video Game Villains.
Source: Wikipedia, "Characters in The Legend of Zelda series," available under the CC-BY-SA License.