SPOILERS!
I voted for Impa, although I'm a little iffy on whether or not she's a minor character. Impa is the first character you meet when you first come to the Surface. She was there during critical story moments, such as when The Imprisoned broke out (all three times). Don't forget that she's also the one to have protected Zelda for thousands of years, as well as being the one to guide her to certain places around the Surface.
I'd say she's important as Groose, really -- not really a minor character. She was a crucial part of the story who appears much more than once. The other characters that were listed can easily be labeled as minor since they have very little to do with the story (Karane doesn't really do anything, for example), but I think that Impa doesn't fall into that category.
Especially since she was there with Link, Zelda, and Groose in the ending cutscene (before the credits).