He's the commissioner's stepson and started his journey at the 15 division in season 5. Andy McNally was his training officer. He screwed up on multiple occasions but Andy wanted to keep believing in him and she did until he bailed on her in the middle of a dangerous situation. She was stuck in a house with a small child and a gangbanger trying to kill the child. She had to report him and deemed him unfit for duty. He was suspended and ended up filing a case against Andy. It was eventually sorted out but he got to return to duty.
In 74 Epiphanies, Duncan stated to Andy that he is leaving 15.