Sometimes to understand a child, one might need to understand the parent. That was most definitely Harry Potter's case when he started to learn about his father. James Potter can be considered a plethora characteristics both good and bad. We never actually meet James Potter until book five (The Order of The Phoenix) and even then that was in a biased memory from Professor Snape. So, during books one through four, Harry first grows up having no idea who his father is and then overnight (literally) is told that his dad is a hero in the wizarding world. For a lonely orphan, that can be the best description a child can get about his parent.
As the books progress, Harry begins to delve deeper and deeper into who his father was. Upon meeting his godfather, Sirius Black and Professor Lupin, James' oldest and closest friends, Harry discovers who his father was in a much closer sense. James Potter was a Pureblood wizard from a large and lively family. He was somewhat spoiled, very intelligent and a troublemaker of the highest extreme, However, his trouble doesn't always com from malicious intent (that's what Harry is led to believe in the beginning), but rather to help others when no one else will.
However, not everyone is a fan of James Potter. While James was a good friend, he was a terrible adversary especially toward Severus Snape. When Harry was younger, Snape oftentimes told him how his arrogance, stubbornness and disregard for the rules came from his own father. How James ruined his life and made it miserable. Of course, Harry doesn't believe it until he gets a cold dose of reality when he discovers 'Snape's worst memory' is of James and his friends hazing and bullying Snape when they were at school together.
As a reader who (at the time) hated Snape, I both loved and hated the chapter. It was nice seeing the snarky professor get owned when he was a student. Yet, it also confirmed that everything Snape said about James was true; he was arrogant, self-centered, uncontrollable and a bully. Snape is the way that he is toward Harry because of James' treatment of him in school. However, what was hardest for Harry to believe was that his own mother hated James. For years he thought they had this wonderful perfect love story, but it turned out to be so much more complex.
Harry is comforted by Lupin and Sirius afterwards when they tell him that his father did settle down, left Snape alone and eventually Lily did fall madly in love with him. James finally decided to grow up and realize that there were worse and more evil enemies out in the world then the ones he had at school. James Potter did die sacrificing himself for his wife and his baby and became Harry's greatest inspiration for overcoming the growing darkness in his troubled life.
While I wouldn't say that James Potter is a controversial character, fans have had a lot of fun discussing his character as a student. Some people think he's terrible, but I simply say that he was a regular teenager who had a lot of growing up to do. The best thing about the character of James Potter is that he is only seen through memory and perception. And yet what we do mange to see was a flawed, but very brave (or reckless) boy who developed into an extraordinary man who defied fear and hate, even if it meant losing his life. Maybe James saw elements of his younger bullying self in the evil he was fighting and realized how prejudiced and cruelty can be the beginning of personal destruction.