Princess Ruto

Princess Ruto was a Zora and the daughter of King Zora, who resided in Zora's Domain in Ocarina of Time. She's an adventurous, yet temperamental, young Zora girl who got into trouble when Lord Jabu-Jabu, the Zora's fish-like deity accidentally swallowed Ruto's most precious treasure, the Zora's Sapphire. Angered, she managed to get inside Lord Jabu-Jabu's belly without telling anyone, causing distress among her people and father who were unsure of her whereabouts.

When she realized she couldn't get out either, she sent a message inside a bottle that ended down on Lake Hylia, which Link recovered. Prompted by this, Link entered Lord Jabu-Jabu's belly to rescue her. However, Ruto refused to leave until she could find her treasure. When they finally recover Ruto's belonging, Link realized that's the jewel he'd been looking for: the Spiritual Stone of Water. Ruto was then subsequently captured by a Big Octo and Link was forced to continue through the dungeon. Once he defeated Barinade, he was reunited with the Zora princess.

Ruto, having grown fond of Link, agreed to hand the Stone to him on one condition: Link would have to marry Ruto when the time came. This was because Ruto's late mother made her promise that she would give the Stone only to the man that would become her husband. Having no other option, Link received Ruto's memento as a sign of their engagement.

After Link was sealed by the Master Sword inside the Temple of Time, Zora's Domain and its habitants were eventually frozen by the evil that flowed from the Water Temple when Ganondorf acquired the Triforce of Power and conquered the Sacred Realm and Hyrule. Soon after Link was freed from the Master Sword's seal, Sheik managed to free Princess Ruto from the ice, who in turn went immediately towards the Water Temple to try to cast away the evil from the temple. When Link found her once again inside the temple, they had a short reunion recalling their engagement; nevertheless, Ruto declared that it's no time to talk about love, and offering her help, she requested of Link to destroy the evil the temple suffers from in order to defrost Zora's Domain and save her people.

Once Link achieved this by destroying Morpha, Ruto is revealed as the Sage of Water and speaks of her feelings for Link, but, given it's still not the time, she postpones their engagement and prompts Link to continue his quest of awakening the remaining sages. She finally asks Link to give Sheik her thanks for rescuing her, which he does.

She, with the other six sages, sealed Ganondorf inside the Dark Realm at the end of Link's final clash with Ganon.

Miscellaneous
Apparently, Ruto has the ability to read people's minds. That, or she's quite apprehensive to other people's thoughts.

Princess Ruto, like several other characters from Ocarina of Time (including Saria and Mido) was named after a town (called Ruto) in The Adventure of Link. (Or rather, the town was named after her.)

In The Wind Waker, Ruto is depicted in a stained glass window inside the Master Sword's room with the other five sages from Ocarina of Time.

In the Phantom Hourglass, one of the available treasure items is titled "Ruto Crown". The description states "Some say this regal crown was worn by a princess of Zora". This becomes the second reference to her after the fall of Hyrule beneath the Great Sea.