The spirit who lives in the sword in Skyward Sword is Fi. She first resides in the Goddess Sword, the divine weapon that Link later transforms into the Master Sword.
Fi serves as Link’s guide and companion throughout the game. Created by the goddess Hylia, she was designed to assist the chosen hero in defeating Demise. Her logical, formal speech and highly analytical personality deliberately contrast with Link, who is a mostly silent protagonist.
Link strengthens the Goddess Sword by gathering Sacred Flames and completing trials connected to the ancient goddesses. After the blade is upgraded into the Master Sword, Fi remains its spirit. At the end of the adventure, Link seals Demise’s essence within the sword, and Fi enters an eternal sleep inside it.
Sheik, Midna, and Navi are also notable Zelda characters, but each belongs to a different story role. Sheik is Zelda’s disguised identity in Ocarina of Time, Midna is the imp-like companion in Twilight Princess, and Navi is Link’s fairy guide in Ocarina of Time.