Impa is a jerk, Fi is pretty cool.
But that is just how I feel about them. ^^
The first time in SS I met Impa, after running full speed through a lava dungeon, running from a boulder Indiana Jones Style, and finally fighting Lava Kirby, she comes up to me and says 'Why weren't you faster? If I didn't use my teleportation powers and SKIP this place, Zelda might have been in danger. Where is your teleportation powers boy? Go home.'
And I don't hate fi because she never got on my nerves. Seriously, I find like 90% of the other characters in the game WAY more annoying then Fi ever was to me. ^^