[Shrugs] In mine opinion, any character is mainable, given enough practise and determination.
Anyhow, I can use Palutena pretty well, but I stopped maining her due to many bad matchups. Palutena is basically a defense based character with few approach options, and as a result she is very hard to use aggressively without taking significant risk. I am by no means an aggressive player, but frankly, there are times when an offensive approach is needed if you don't want to get mauled.
If I can think of one character at the top of mine head whom is really hard for me to use, its definitely Sheik. Despite largely being considered the best character in the game, she doesn't feel natural to me at all.
[EDIT] Just realised mine comments didn't totally relate to the thread. Besides Robin, I really wanted to main Shulk, but in the end he just wasn't for me. Though, I still use him for fun on occasion.