It's part of her mission

Link locked the door behind him as he stepped back into his room. Even after the eventful night he had, being assigned with the mission of bringing Zelda back and being granted a magic sword on top of it, he still had to stifle a yawn as he shuffled his way to bed. He gingerly unslung the sword from his back as he sat down on the bed. He leaned the blade against his desk as he laid back, ready to fall asleep. That was until he heard a sound emit from the sword. He shot up in bed as he realized that Fi had reappeared, floating near him. She looked down at him with an emotionless expression, "You are still awake master Link." She said.

Link sat up, he didn't know how to feel about the spirit inhabiting the sword yet. But, whatever she had to say must be important. "Do you need something Fi?" He yawned despite his efforts to appear attentive.

The spirit nodded, "There are matters of importance that I must discuss with you. I apologize for not informing you earlier, but for the sake of ease I waited until we were in a more private area."

Link nodded, ever more convinced that whatever she had to say must be really important if she needed privacy to talk about it. "Go on."

"As part of my function as your guide, I am capable of changing my appearance to be more appealing to you master. This is merely my default form, but if you so wished I could change it." Fi spoke as if this were just as important as everything else that her mission entailed. "If you need examples of what kind of changes are possible, I can change breast size, hair length, body proportions, clothing, to name a few."

Link blushed at what she last mentioned, "Wait, did you say clothing?"

Fi nodded, "Yes, if you so wished I could change my garb into something else, or I could remove it entirely."

Link's blush deepened, "N-n-no, that's okay. You don't need to do that." He felt as Fi looked at him quizzically.

"My calculations say that there is a 97% chance that you are lying. As such, I must insist that I be allowed to remove my clothing."

Link was embarrassed to say the least, he'd never really had any kind of sexual experience like this. Yet here was a beautiful spirit begging him to let her take her clothes off. He weighed his options, worst case scenario he could just ask her to put clothes on. So he didn't see much harm in just giving her what she asked. "Well... if you insist."

Fi lightly bowed her head, "Thank you master." She disappeared, her spirit returning to the sword and she reappeared a few seconds later. Link was taken aback. The simple purple dress she wore was gone and he could see the gem in her chest embedded just above her perky breasts. Even the cloth on her shoulders had been replaced with two delicate and slender arms, the same blue as the rest of her now naked body. However, despite Fi's insistence that she was getting naked, there were still two pieces of clothing that remained. First were her stockings which ended about midway up her thighs. Second was a purple choker around her neck which accentuated her state of undress.

Though she didn't express it outwardly, Fi seemed to be pleased. "You seem to appreciate this form master." She turned around, striking a pose that showed off her ass and the curve of her back. "Please get a full view before you make any decision on this form." Link felt himself grow even harder as he looked down every curve of her body. To his surprise, he could see that she had what appeared to be normal female genitalia. Maybe it was there for aesthetic reasons, he wondered, she did say that she could appear however she wanted.

Fi turned back around, she grabbed both of her breasts, pushing them together while bending over towards Link. "If it pleases you master, I can change the size of these to your exact specifications." Link barely heard her, the carnal display in front of him made sure of that.

Fi continued to list of possibilities as she struck numerous lewd poses, bending over to show off her ass, pushing her breasts up with her hands, even floating upside down to show how her breasts could follow gravity in certain ways. "I can also modify my appearance to make it seem like my skin is covered in an substance like water or transparent oil, my data shows that the appearance of moisture on the body can improve its appeal to an observer."

Link was already so horny from this display, the thought of it getting any more lewd had him ready to burst. But that was when she listed her final features.

"And of course, I am equipped to perform a variety of sexual services up to and including: anal sex, oral sex, vaginal sex, and other forms of sexual stimulation."

Link's heart jumped, not only was she showing off her body in every way possible. She would also let him use her body in whatever way he wanted.