I agree with TB's assessment of Fredo. He is stupid and weak and probably couldn't find his way out of a paper bag if Sonny and Vito weren't at the other end with a flashlight. The one thing that was obvious was that he loved his father, as we can see by his complete emotional meltdown at his father's shooting. This might lead people to comment how good-hearted he was.

However, he does nothing to show that he's selfless or has a good heart. He doesn't learn the casino business. Instead, he prefers to use his position as a Corleone to have sex. He marries a drunken tramp, he frequents sex clubs in other countries, becoming nothing like the father he supposedly adored.

As a matter of fact, Fredo shows anything BUT a good heart. If his actions are to speak for him, once he leaves his father's house, he leads nothing but a completely hedonistic life, concerned only with his own pleasures.