Kokiri Sword

The Kokiri Sword is the default sword Link possesses in both Ocarina of Time and Majora's Mask. It is also the weakest sword in both games.

Ocarina of Time
In Ocarina of Time, Link must obtain the Kokiri Sword before Mido allows him to visit the Great Deku Tree. The young hero can find the Kokiri Sword hidden away in a chest inside the Hole of "Z" in the Forest Training Center. This is the only sword Link can wield as a child, and when he is an adult, he uses the Master Sword instead.

Majora's Mask
The Kokiri Sword is equipped by default at the beginning of Majora's Mask, and is the same Kokiri blade which Link used in Ocarina of Time. It is hindered both by a short range and a lack of power, but can be upgraded to the temporary Razor Sword and then upgrade again into the permanent Gilded Sword by taking it to the Mountain Smithy in Mountain Village. Although Link begins the game with the Kokiri Sword, he is unable to use it when he is first transformed into a Deku Scrub by the Skull Kid.

Interestingly, the hilt and guard of the Kokiri Sword changed from Ocarina of Time to Majora's Mask.

Ocarina of Time Manga
In the, Link receives the Kokiri Sword from Mido after they defeat Gohma. It is also used to carve a Deku Shield out of the Great Deku Tree's wood.

Theories
Some have noted that the Kokiri sword looks much like the Four Sword. It is possible that they are the same, for the following reasons:
 * 1)  The Kokiri and Picori are very similar, and could very well be the same race.  They are both often related to the forest and nature.  Also, they are both very elfin, having pointed ears, a mouse's smile, and Robin Hood style hats.  Above all, the Kokiri/Koroks have been known to change into very different races over the centuries, and it is possible that the Picori became the Kokiri.
 * 2)  Both the Kokiri Sword and Four Sword/Picori Sword are treasures of the tribes possessing them.
 * 3)  They look very similar, both having similar hilts and similar blade length.
 * 4)  They are both wielded by a child Link.

However, there are some facts suggesting that they are NOT the same:
 * 1)  The Picori are known for their small stature, but the Kokiri are not small at all. Although it has been proved in The Minish Cap that at the time of the Picori Festival, humans and Picori, like Link, Ezlo, and Vaati, are able to go between the worlds using stumps/jars.
 * 2)  The Kokiri sword does not seem to have any kind of magical ability, but the Four Sword does.

Trivia
The Kokiri Sword appears in its Ocarina of Time incarnation as the blade wielded by Young Link in Super Smash Bros. Melee.