I'll be that one guy who goes for Lulu. Lulu isn't underrated in battle nor is she underrated on the looks department but her backstory needs more credit. I only really learned just how good it is when I read a webcomic about it which was faithful to the lore. Excellent. I guess it's not quite canon but it swings this for me.

Edward just annoyed me too much. If he was actually really awesome then I'd say he was underrated, but in my opinion his character warrants the rating.

Note: Haven't finished FFIV (got about half way - but have read full Let's Plays) so can only go by what I know.