Dark Link

Dark Link (also known Shadow Link or Link's Shadow) 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 final boss in the game, he proves the most difficult opponent during Link's second quest to save Hyrule. He dodges most of the attacks that Link attempts, but he 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 the Biggoron Sword, or by using  his sword and while L/Z targeting him do a horizontal slash on the opposite 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. However, once he is given enough damage, he will fight on his own, countering every sword attack. Din's Fire is the best option in this part of the battle. Once Link defeats his alter-ego, he is able to continue on to a room containing the Longshot. This Dark Link was created by Morpha.

In Four Swords Adventures, Dark Link yet again makes an apearance; this time by the name Shadow Link. He proves to be quite persistent, and is featured as a mini-boss several times before he is finally felled.

No one knows where Dark Link's origins are, but many speculations are out there. Some say that he is the ghost of a past warrior that is related to Link. Others think that, in a sort of dead-man-switch technique, Dark Link was created by Ganon himself as insurance, should he fall to Link's blade. Some say, just maybe, Dark Link was created out of Link's heroism. Because Link is the hero that he is, Dark Link arose due to complete Link's dark side. Everyone has a good and bad side to them, (Yin and Yang) thus, it is a widely accepted theory that Dark Link is the force that balances Link, and possibly keeps him alive. Regardless, it will be known in due time.

Dark Link also appears in one of the challenge scenarios in Super Smash Bros. Melee, where you must defeat him with Link.

Part of Dark Link's popularity is the sheer mystery behind him. Who is he? Why is he intent on combating Link? If, indeed, the before-mentioned theory is true, then, does Link have a dark side to him alone? Is he capable of doing no wrong?

Fierce Deity
Some people believe that (Fierce Deity) Link is Dark Link in another form you can take control of.

Bongo Bongo Theory
If the Timeline Theory is correct, and Ocarina of Time is the first game in the Timeline, some people on a Forum think that Bongo Bongos "Smoke" Form (That escaped from the Well) only showed his true form when he flew through Link, thus removing all evil from Link. And when Bongo Bongo was destroyed, Dark Link was born. This is stopped by the fact that Dark Link is in the Water Temple, and you cannot get to Bongo Bongo (Shadow Temple) until you've completed the Water Temple. Maybe it is that after Dark Link was defeated, Link was unbalanced and Bongo Bongo's smoke form restored the evil in Link, thus balancing him once more.

Descendent of the Twili?
In Twilight Princess, the Light Spirit Lanayru tells Link a story warning him about the corrupting power of the "Fused Shadows", as well as giving their origins. As part of the visual aid, Link and his childhood friend Ilia represent corruptible humans, while the corrupt sorcerers who would become the Twili are represented by three copies of Link with colorless clothing, black skin, and glowing, red eyes. It is debatable whether these Dark Links are merely a representation of evil or are the real descendents of the Twili (in which case, Link would be distantly related to the Twili).

Manga
In the manga, Dark Link shows up in the well instead of Bongo Bongo. It is actually just a shadow beast that can take the form of anyone it chose, creating havoc in Kakariko Village. When Link fights him, Dark Link repeats exactly what he does when finally Link does a backflip, that he just learned from Impa, over Dark Link and defeats him. Then the shadow transform into Link again, but this time on a "Dark Epona". Link easily defeats him and thats the end of the shadow beast and Impa tells Link that she is one of the Sages. There is no Bongo Bongo in the Manga.