Fi, is a character from The Legend of Zelda: Skyward Sword. She is a fairy spirit residing within the Goddess Sword, created by Hylia to guide her chosen hero on his quest to defeat the Demon Lord. Her manner of speech is modeled after that of an artificial intelligence as they commonly appear in fiction; her dialogue is represented by audible, but incomprehensible, Auto-Tune vocalization. Physically, she has a humanoid feminine figure despite being genderless. She bears a resemblance to the Queen of Fairies from The Legend of Zelda: The Wind Waker and The Legend of Zelda: Four Swords Adventures; the significance of this, if any, is not elaborated upon.
Created by the goddess Hylia untold years before the events of Skyward Sword as part of a two-part fail safe against Demise, Fi is the humanoid representation of the Goddess Sword. When the time comes for the Goddess's chosen hero, Link, to set out on his quest to stop Demise, Fi appears in one of his dreams, and once he awakens, guides him to a hidden chamber found inside the Statue of the Goddess in Skyloft. There, Link uncovers the Goddess Sword; once he wields it in his hand, Fi enters Link in her registry as her "Master".
Throughout his adventure, Fi aids Link in a multitude of ways: these include giving sage advice about the current situation, summarizing the story for him in broad strokes, and providing him with information on nearly all people, enemies, and objects that can be Z-targeted. She will also notify him when his Heart Containers, Shield Gauge, or the batteries in the Wii Remote are nearly depleted. She also allows Link to use dowsing to find certain targets based on their energy signature. Whenever Link must play a song on the Goddess's Harp to advance in the game, Fi sings along with him to cause the desired effect of the song.
Speaking to her while low on hearts will unlock the ability to dowse for Hearts.
At one point, Fi uses her "powers of persuasion" to convince the reluctant Scrapper, an Ancient Robot, to aid Link in retrieving a missing propeller needed to restore functionality to one of Skyloft's windmills. Scrapper, who refers to her as "Mistress Fi", aids Link on several other occasions, but claims to be doing so only to please Fi. Link and Fi are separated on several occasions; when Link enters a Silent Realm, she cannot follow him on his journey of spiritual advancement, rendering him defenseless against the Watchers found there. They are also briefly separated when Link is imprisoned and relieved of his equipment at the Eldin Volcano.
Eventually, after Link imbues the Goddess Sword with all three of the Golden Goddesses' Sacred Flames, Fi's vessel is transformed into the Master Sword. Once she has successfully aided Link in defeating Demise, she asks of him to return the Master Sword to its pedestal; however, doing so will cause Fi, her purpose fulfilled, to fall in a deep slumber. In her final moments before entering an eternal slumber to seal away the remains of Demise's consciousness, Fi tells Link how she learned much from their time together and thanks him wholeheartedly with the hope that they would one day meet again.