Fredo? I think he is the worst character, i don't know if he has a character at all =) He even couldn't hold the f*ing gun when they killed Vito, he made a move to Kay in the first movie when Michael introduced her if you remember which i can't understand, Kay didnt understand too. He became against his own brother in the Moe Green incident. What a brother.. Then of course he sold all the family backstabbing them when they were trying to be legitimate. Yes, it was a sad moment when he died (mostly because of music and the heaviness of the scene :) but its not enough, nope.. i wouldnt even feel his absence if he didnt exist at all in the story.
Michael f*ked up things so bad too..I will second Vito :) On the other hand I like Kay also.