Sheik

A mysterious warrior with great wisdom and strength, Sheik is one of the last surviving Sheikah in Ocarina of Time. He[0] aids Link on his quest without really describing who he is or why he is aiding Link. He has great advice, wisdom, and knowledge of the world, making him an invaluable ally for Link. Sheik is also the teacher of the various warp songs that Link uses later on in his quest.

All of the quotes from Sheik can be read here

Ocarina of Time
Sheik reveals himself to Link shortly after Link is freed from the Chamber of Sages after his seven year imprisonment. He informs Link that he must awaken the seven sages in order to bring peace back to Hyrule. Alerting him of the need to possess a new tool in order to enter the temples, Sheik stays in front of the pedestal. He then later appears when Link arrives at the Sacred Forest Meadow. He tells Link about the warp songs used to warp to temple entrances and teaches him the Minuet of Forest before disappearing again. Further along, Sheik approaches Link at the Fire Temple entrance in the Death Mountain Crater, teaching him the Bolero of Fire before walking away, using a wall of flame to halt Link's inquiring advance.

Sheik once again mysteriously appears in the Ice Cavern to inform Link of the fate of the Zoras and to teach him the Serenade of Water. He then awaits outside the Water Temple for Link to free the temple and awaken the Water Temple sage before disappearing once again.

When Kakariko Village comes under attack from a mysterious force, Link discovers that Sheik is already there. Together they encounter and are defeated by an invisible monster thought to be the Shadow Spirit. Sheik teaches Link the Nocturne of Shadow and urges Link to help Impa fight the Shadow Spirit. When Link discovers that he must venture back in time in order to do so, Sheik offers a helping hand informing Link on how to use the Master Sword and the Pedestal of Time in order to travel back and forth through time. He teaches Link the Prelude of Light then once again vanishes.

At the entrance of the Spirit Temple, Sheik appears to have been waiting for Link from atop a large rock formation, and once again, informs Link that he must enter the temple as a child, utilizing the Pedestal of Time in the Temple of Time. He gives Link a lesson on performing the Requiem of Spirit before using the desert winds as cover to vanish. 

With all six sages freed, Link is instructed to return to the Temple of Time where none other than Sheik awaits. Sheik congratulates Link on his efforts and offers an explanation on what exactly happened to Hyrule during Link's disappearance before finally revealing his true identity, as Princess Zelda.

Twilight Princess
According to the Super Smash Bros. Brawl official site, a design of Sheik was drafted as he might appear in Twilight Princess, but this idea was later scrapped. His Twilight Princess model was used in Super Smash Bros. Brawl.

Super Smash Bros. Melee
In Super Smash Bros. Melee, Zelda has the option of transforming into Sheik. To do this, you must press down the analog stick while pressing the B button. Sheik has a completely different moveset than Zelda. Sheik's moves and weapons are quick and similar to those of a ninja. Sheik is an extremely speedy character and for this reason he could be a bit difficult to control. Although he has an original moveset, she is found to have similarities with Captain Falcon. In Melee, the two of them have a more powerful star rod attack. In the game, Sheik is always referred to as a female.

Super Smash Bros. Brawl
Sheik appears again in Super Smash Bros. Brawl. Although he did not appear in Twilight Princess, the character model was based on a design drafted during the game's development. He has mostly the same moveset from Melee, and shares a Final Smash with Zelda, the Light Arrows.

Ocarina of Time manga
In the, Sheik first appears to Link having tended his wounds after being defeated by Volvagia. Sheik is a spy for Ganondorf and later shows up to tell Link a rumor that Princess Zelda had been spotted in the Desert of Illusions, before vanishing again. On Link's way through the Desert, he nearly succumbs to blistering sands before Sheik pulls him out and continues to travel with him. Later that night, Link is attacked by illusions and images of his past, but Sheik is unaffected; Link wonders if it could be due to a lack of past in Sheik's life. Once reaching the Gerudo Fortress, Sheik knocks Link out, and the Hero is captured.

Once Link frees himself, he fights through the ranks of Gerudo, before Sheik comes to him with his sword and shield. Throwing them to Link, Sheik attacks the invisible Twinrova, who were watching nearby. They accuse Sheik of being a traitor. Sheik replies," A traitor? I was never one of you!" The Twinrova then blast Sheik into unconsciousness.

After escaping with the unconscious Sheik, Link finds a cave to rest in. He leaves to find water, flustered after getting too close to Sheik. He returns to find that Sheik, having awoken with the face wrappings off, appears to be female. Fully transforming, Sheik is revealed to be Zelda, who later explains that she had Impa lock her consciousness away and assume the disguise of a Sheikah boy.

Debate on Sheik's sex
Sheik's gender is one of the most controversial subjects within the Zelda fandom, the mere mention of it enough to bring about long and heated debates. Some will vigourously insist Sheik is a female due to being the alter ego of the female Zelda, while others will vehemently insist that the primary purpose of Sheik is not being Zelda, so the latter won't be hunted.

Sheik has canonically only appeared in Ocarina of Time. Within that game, Sheik's gender was referred to once, by Princess Ruto, and she referred to Sheik as a "a young man" and "him". The Official Nintendo's Player Guide describes Sheik as "a young man" too and Sheik's art displays a significantly more masculine build than Zelda's. Therefore, it would seem that Sheik within the context of Ocarina of Time at the very least is supposed to be perceived as male, regardless of what "he" may truly be. For that reason, the Zelda Wiki has opted to refer to Sheik as male, because the perception is the only certainty about the character's gender.

As to what Sheik's true gender is, the sources are contradictory and what significance a source holds is up to debate. Within the game, Sheik is never unambiguously indicated to be of either gender. "He" may very well be merely a well-done disguise that had Ruto fooled into thinking "he" was a guy. And in that vein, the Official Nintendo's Player Guide is not unlikely written as it is in order to not spoil the revelation of Sheik's true identity to the readers.

The stance that Sheik is a male is supported within the Ocarina of Time manga, in which Zelda used magic to become a Sheikah boy to be safe from Ganondorf. Though the manga contradicts the game on various points, Eiji Aonuma has said that "the manga makes up for [the game] by filling the gaps "precisely"." Alternatively, the Super Smash Bros. series follows the stance that Sheik is female, referring in the trophy descriptions to a "her". While a sub-discussion may exist over whether or not Brawl Sheik counts as evidence, as she is the non-canonical alter ego of the Zelda from Twilight Princess, Melee Sheik ís based on the Ocarina of Time version ánd also called female. She retains the masculine appearance of her Ocarina of Time self.

A sort of third option to the debate is that Sheik originally may have been meant as a male character, but has been recast as a female in recent years to better appease to the market. As the debate proves, the idea of gender as an inherent and vital component of an individual's being is deeply ingrained in the (Western) mindset. Nintendo has over time made Ocarina of Time less controversial, such as by recoloring Ganon's blood from red to green, removing real-life religious symbols, and for what its worth, writing the ReDead trophy description in Super Smash Bros. Melee to destruct the implication that the ones in the Hyrule Castle Town ruins were some of the previous inhabitants. It may be that Sheik as a secret male alter ego to a well-known female character originated as a creative story element, but that such a character would be difficult to market as a multiple times genderswapping combatant in the Super Smash Bros. series. Also, Nintendo does not have a large cast of noncontroversial, famous female characters, which makes a female Sheik more interesting from a marketing perspective.

Trivia

 * In the game , released two years after Ocarina of Time, there is a Little Mouser king named Moustafa. To keep himself from being found, he disguised himself as a information dealer named Sheek, most likely a reference to Sheik.