Every boy and girl in Kokiri Village have a fairy except for one young boy named Link. As the great Deku Tree senses a dangerous threat coming to Hyrule he sends out Navi to aid Link on his journey to save Hyrule. Navi being unsure about this as Link is a small boy and not knowing if he can handle this journey or not doesn't question this to the Deku Tree as she accepts her duty.
At the beginning of Ocarina of Time the two didn't get along until Link wakes up from deep sleep slumber after seven years. Then the two practically become best friends in fact so close that in Majora's Mask during the beginning Link is looking for Navi.
In Ocarina Of Time Navi usually tells Link where to go or what to do while she's doing neither she says her catchphrases "Hey!" , "Look!" , and "Listen!". These catchphrases tend to annoy the fans until at the end of the day they remember how important Navi is. At the end of Ocarina of Time she flies quickly inside of the Temple of Time then the screen goes into another scene. It's unclear if Navi stays in the same realm or goes into a different world. Though some people wonder if she'll ever be a main character in the series again as she's well known.
APPEARANCES AND REFERENCES
Ocarina of Time
Link And The Portal Of Doom (book)
Possibly referenced in Majora's Mask
Four Swords Adventure (mini game named after her)
Wii cursor in Twilight Princess
Smash Bros. Brawl is one of Link's taunt.