Yeah I kind of fail to see what isn't human about it, or why she'd have another form at all.Fi's "human" form is exactly how she is
Either way, the notion of her having another form makes no sense, and also operates under the assumption that Ghirahim and Fi are exactly the same, which is completely false. Same idea, but two different creatures with completely different origins and natures. Fi has absolutely no reason to have another form, and if she did Link would have been irrelevant. You also need to remember that Ghirahim is a demon of some form.
If you're still not convinced that their natures are completely different, then consider this: Fi emerges from within the Goddess Sword. That's where she comes out of and where she goes back into. At the end of the game Demise pulls the sword out of Ghirahim, not to mention he suddenly shifts back into it briefly during the conversation before the final battle with him. Fi is just the sword's consciousness. Ghirahim is a physical avatar of the sword.