Fi

Fi was a figure of amazement.

She was a spirit inhabiting the Master Sword. One could even go so far as to call her the Master Sword itself!

She was created by the Goddesses for the Hero, the very first Hero, to find and use when the demon who threatened the world came to finish the job. She was meant to remain dormant until the evil revived and the Hero came to her, but Cia had opened a portal to Skyloft at a point before the realization of the prophecy. Still, the presence of monsters and a Link from the future was enough to awaken her, and she decided she herself would fight when and where she could.

Even when manifested it was clear she was a spirit and not mortal. Her entire body, flesh and hair and all, was made of a blue glasslike material, she was cool to the touch, and her eyes were blank and featureless. Her feminine form was tiny and slender, her chest small under the large pointed diamond on her front.

Still though, as an autonomous holy weapon she was formidable, and she was singleminded in her service to the fight.

Fi was kept in Link's own tent when not in battle, dormant. She said that she was created for the purpose of serving Link in his trials against evil, perhaps if he were to order her to please him sexually, she would? Though there's a good chance she had no knowledge of sex, in which case he would have to impress on her body what the act meant.