He was the war hero in the family, the most respectable son of Don Vito Corleone, who had the highest hopes for Michael in the “legitimate world.” But when his father, Don Vito Corleone was shot, Michael couldn't keep from getting involved in the family business. He wound up taking over. What followed, was his freefall into a bottomless pit of corruption. While he once distanced himself to Kay from the ways of his family, Michael eventually grew to become the worst of everything his family represented. Even still, he loved his family dearly. Family was the most important thing to him. However, he was so vengeful and power hungry, he killed his own brother, Fredo.
He was sane and mad, kind and cruel, powerful and weak. He was a masterful, strategic thinker blinded by vengeance. He would publicly renounce Satan and all of his works as his godson was being baptized, and promise that he would protect that child from the wickedness of the world, while outside his men murdered all of his enemies.
And then in that one critical moment when he agreed to be honest with Kay and tell her about his business, he lied to her. The main character, Michael Corleone, played by Al Pacino, is a prime example of tragedy in a hero. And one of the reasons he is such a good, deep character, is that he changes tremendously during the film. At first, he is an honest looking guy, a war hero, who is shown to be distant from the rest of the family, but still loved and included by them. Then due to certain events, he morphs into the ultimate criminal.
Michael Corleone is in my opinion, the main reason the Godfather Trilogy was so popular and made such an impact in the film industry. That shows how important even one character is to a movie's success or failure.