Dark Link
Dark Link (also known Shadow Link, Link's Shadow or in the Japaneses versions Doppelganger) 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).
Dark Link's Incarnations and Appearances
The Adventure of Link
In his first appearance in the series Dark Link appears as the leader of the Cult of Ganon and the final boss in the game, he serves as the final test before Link can obtain the Triforce of Courage and wake Princess Zelda. Dark Link is the most difficult opponant to appear during Link's second quest to save Hyrule. In battle he dodges most of the attacks that Link attempts but is vulnerable when he jumps. This flaw can be taken advantage of by simply standing in a corner and repeatedly swinging Link's sword toward Dark Link. Dark Link will attempt to jump at Link and keep running into the attacks making a rather difficult battle seem, less difficult.
Oracle of Ages
During the final battle, Veran created red-eyed copies of Link that moved opposite him as obstacles. These Shadow Links did not attack, however.
Ocarina of Time
Appearing as a mini-boss in the Water Temple Dark Link 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. Whatever you do do not perform a sword lunge as Dark Link will simply jump up and stand on the blade before attacking. Once Dark Link is defeated Link is able to continue on to a room containing the Longshot. This Dark Link was created by Ganondorf as a mirror reflection of whomever entered the temple.
Four Swords Adventures
In this game Dark Link is refered to as 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. Yet another theory is that, since Link needed to be born immaculate (sin-less) in order to touch the Master Sword, the gods extracted his dark side from his soul, leaving only his light; his dark side is Dark Link, whose ultimate wish is to reunite himself to his holy counterpart. 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?
Dark Mirror
The Dark Mirror in Four Swords adventures is that games source of all the shadow links, it is said that one can look into the mirror and pull out their dark reflection. Ganon obviously did this, and stole the mirror, because all of the Links came from this source.
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.
Ancestor 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 ancestors 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 chooses, 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.