Fi (Total War)

Fi is the sword spirit within the Master Sword, she was Hylia's second attempt following the failure of Ghirahim.

The Return of Sulkaris
Hylia created the Master sword for her personal knight Link. However the spirit turned out to be selfish, independent and unstable. So Hylia viewed this as a failure and so sealed Ghirahim within Vigjaro Cathedral, and then creates Fi to be his successor.

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.