I am firmly a fan of Fi.
She's rather straight and logical, like a computer program, but that's part of what I love about her: If you answer "wrongly," she's so deadpan about making you feel silly it's wonderful. She also can convey to me all I need to know about an enemy and how strong I am against a species. She is also a fine lady who should pay more attention to poor Scrapper XD.
I did not like Ghirahim, to tell the truth. He's the kind of creepy, over the top "psychological warfare" villain that I've seen done better and much creepier elsewhere. He gave me too much of a feeling of "Narm," for those of you who visit TV Tropes, but not enough "Narm Charm" for me.