Both characters played a much more central role than I ever expected them to in the story. Upon the first unveiling of the promotional artwork for Skyward Sword, then without a subtitle speculation was rampant on the main site (likely the forums as well but I hadn't joined here yet) regarding similarities between Fi and the Fairy Queen. No one expected her to be the spirit of the Master Sword. Ghirahim, on the other hand, was a demon I suspected to be fight only twice and then fade into insignificant oblivion. I was therefore surprised as more trailers released that he was cast in the spot light. Ghirahim was a refinement of the "pawn" villain concept introduced with Zant in Twilight Princess and even transcended those bounds due to wishing to revive his master on account of his own free will.
That said, I prefer Ghirahim over Fi. After Midna's stunning character development it was hard to care for Fi. Although she finally understood the capacity of human emotion at the end of Link's epic journey, Fi was a sterile robot for most of the adventure. Midna, conversely, transformed from a selfish imp to someone willing to risk her life for the sake of an entire kingdom. It took bravery and a heart of gold to confront Ganondorf upon the defeat of his beast form. A final emotional moment was cast with the breaking of the Mirror of Twilight, a true and well rounded companion vanished into the abyss. As aforementioned Ghirahim was atonement for the weak persona that was Zant in Twilight Princess. Don't get me wrong, the Usurper King certainly carried an aura of mystery but his sudden breakdown in the Palace of Twilight was completely unexpected with no prior narrative buildup. Ghirahim symbolically transformed into his true demon form with his darkening skin hue and with respect to a sky analogy could be interpreted as the darkening thunder clouds during a storm.