Fi (Total War)

Fi is the spirit residing in the Master Sword, the weapon hand-crafted by the Goddess Hylia for her knight Link to use.

The Return of Sulkaris
Following the Gerudo Wars, the False Goddess Hylia gathers the Sages and explains her desire to craft the Master Sword - an ultimate weapon for the First Knight Link. After the first spirit bound within the sword breaks free and has to be discarded, Hylia abducts Fi from the Sacred Realm and forcefully binds her to the Master Sword. Although Fi puts up a fight she is unable to resist Hylia and is sealed within the blade. With the Master Sword successfully completed, Link is unleashed upon Hyrule.

In-game
Fi (and her counterpart Ghirahim) are the single most powerful units in the game, and act as veritable one-man (or genderless robot sword) armies.