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 unexpectedly Lord Jabu-Jabu, the Zora's fish 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 though, causing distress among her people and father for her whereabouts. When she realized she couldn't get out either, she sent a messages inside a bottle which ended 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, having grown fond of Link, agreed to hand it to him on one condition: Link would have to marry Ruto when the time came. This because Ruto's late mother made her promise 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. 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 request 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, Ruto's revealed as the Sage of Water and speaks 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.
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.
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.
Trivia
Princess Ruto, like several other characters from Ocarina of Time (including Saria and Mido) was named after a town (just 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.