...Santino was definitely a complex person in that he had a personality that consisted of emotions and impulses that, while knowning that they were wrong to act on, he allowed to influence his behavior. Santino was built on taking impulsive actions based on his emotions.

Fredo on the other hand was complex in a different way in that he had terrible feelings of guilt because (as pointed out in a post above), he realized that he was unable to protect his father, and he felt guilty when talking to Mike in Cuba, knowing that he was ignorant enough to be fooled into believing Ola and Roth, and that he betrayed his own brother. ...