Michael Corleone's character changes throughout the movie. In the beginning he abandoned his family.
He was dating Kay Adams. Seemed like he was the only Corleone that was living his life to the fullest, by not being a part of his family. Though deep down we all know that he does truly care for his family. I thought he did not want to be part of the mafia, so he was just trying to stay away from them.
After the first 2 hours of the movie you find out that Michael changes, he use to stay away from his family but now he is all about the family. What I am trying to say is he is maturing, he is taking responsibility becoming a true Corleone. This all started when The Don got shot.
He has done so many things that make him a Corleone. He stood up for his father. When Sonny said to him that Sollozzo and McClutsey wanted to have a meeting with them, he came up with this whole plan, for him to kill them. He went through with the plan and he killed a cop something no Corleone has ever done, he also killed Sollozzo. He did it al for his father. For killing a cop and Sollozzo he would have gone to jail for ages. He flees to Sicily for 2 years under watch of a family friend Don Tommasino. In Sicily he lives a wonderful life. He falls in love and gets married to Apollina. A few weeks after the marriage he finds out that his brother Sonny gets murdered from an ambush by the Tattaglia family. Michael and his wife Apollina are getting ready to move to Siracusa, though Apollina gets killed from a car bomb that was meant for Michael planted in the car by one of the Bodyguards.
Michael now acts more mature, me is more responsible. He was trying to take care of his father, now that Sonny is dead when Michael goes back home he will have to take care of his father.