Fi

Fi leads Link to the Goddess Sword which starts their adventure to find Zelda or her grace. Fi is usually hidden in your sword unlike Navi who is usually following you around or hidden in your hat for a moment. Fi plays a very important role as your sidekick.

Fi also has the ablitity to "douse". Dousing is using your sword to locate items, rupees and goddess cubes are an example. At the beinging of the game you can use dousing to locate Zelda and Kikwis.

At the end of the game Fi goes into a never ending slumber and says she and Link will meet in another life and that while travling with you she has experienced happiness.

Theory
When Fi said said she and Link would meet in another life, she may be talking about him and Tatl or Navi.