Saria

Saria is a Kokiri introduced in Ocarina of Time. She is Link's childhood friend from his years in the Kokiri Forest. Among the other Kokiri, she's well-liked. In particular by their boss, Mido, whose dislike of Link might be partially explained by the latter's close friendship with Saria. Saria often goes to the Sacred Forest Meadow to be alone for a while. She plays the ocarina, her most succesful piece being Saria's Song. She becomes the new Sage of Forest later on in the story and helps Link to defeat Ganondorf.

Ocarina of Time
Saria was the first of the Kokiri to greet Link at the start of the story and happily noticed that Link now had a Fairy of his own. Link told her that he was summoned by the Great Deku Tree and thus had to go. Saria was okay with that, but had a feeling that things were about to change. After Link defeated Gohma, she met him at the bridge between Kokiri Forest and the rest of Hyrule. There, the two said goodbye and Saria gave Link her Fairy Ocarina as a memento.

After Link had met up with Princess Zelda and started his quest for the three Spiritual Stones, he returned to Kokiri Forest. There, the other Kokiri told him that Saria was waiting for him at the Sacred Forest Meadow, deep within the Lost Woods By listening carefully to the direction a strange, cheery melody came from, Link was able to find his way through the woods to Saria, who turned out to be the source of the music. Saria was happy to see Link again and invited him to play a few notes with her. Link accepted the invitation and learned Saria's Song. Saria informed him that by playing her song, he could telepathically contact her. Through the ocarina, Saria kept giving Link advice on his next objective throughout both his childhood and adult years.

During Link's seven year absence, Mido kept spreading the rumour that Link was responsible for the death of the Great Deku Tree. Of all Kokiri, Saria was the only one who stood up for Link. When Link awoke, the Forest Temple in the Lost Woods became his first destination after an emergency call from Saria. On the way, he came through Kokiri Forest, where the Kokiri informed him that both Saria and Mido had left to the Lost Woods. Following them, Link first encountered Mido, who refused to let him pass. However, after playing Saria's Song, Mido understood that Link had to be one of Saria's friends and as such could be trusted and let through. Link continued to the Sacred Meadow Grove and discovered that Saria's "secret place" housed the entrance to the Forest Temple. Link went in and, after a lot of exploring and fighting, managed to defeat the temple's evil spirit, Phantom Ganon. By doing this, he allowed the new Sage of Forest to awaken: Saria. Saria said goodbye, knowing that the carefree days of old were over, and gave Link the Forest Medallion.

When Link later talked to Mido again, the latter asked him if he would help him keep a promise to Saria: to tell Link that Saria had been waiting fo him, because she "really liked [him]" When Link even later defeated Ganon, Saria and the other Sages bundled their powers to seal away Ganon. During the ending, she could be seen overlooking a celebrating Hyrule on Death Mountain with the other Sages.

The Wind Waker
While Saria did not appear in The Wind Waker, one of the stained glass windows in Hyrule Castle's basement depicted her.

Trivia

 * Saria, like several other characters from  Ocarina of Time was named after a town featured in The Adventure of Link.
 * The character Ilia from Twilight Princess seems to be a reference to Saria. Both are Link's childhood friends and the names resemble each other (the "R" and "L" in Japanese are the same letter).