Princess Ruto

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

When she realized she couldn't get out either, she sent a messages inside a bottle. This bottle ended up down on Lake Hylia, which Link recovers. 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 finds her once again inside the temple, they have a short reunion recalling their engagement; nevertheless, Ruto declares 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 perceptive and can interract with people well.

Some people believe that Ruto town in the Adventure of Link is named after her.

It is also thought that the Rito tribe's name may have come from her, as the rito are descendents of the Zoras.

Trivia
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.

In The Wind Waker, Ruto is depicted in a mosaic inside the Master Sword's room with the other 6 sages from Ocarina of Time.