Reese Marshall came to Port Charles as an FBI Agent to assist Sonny when his children were kidnapped. At first, Sonny didn't trust her, because she had a lot of information about him and he was sure she was trying to take his organization down. But Reese showed so much heart and compassion towards Sonny and his children's ordeal, that he slowly started to trust her.
When Michael was presumed dead, Sonny was devastated. Reese confided to Sonny that she had also lost her son to violence and that was why she became an FBI Agent. When Sonny's children's kidnapping case was over and they were back in Port Charles, Reese was assigned to take Sonny down. She refused the assignment and decided to start her own law practice.
When Michael returned from his kidnapping ordeal with AJ, he was very traumatized. It was Reese that was able to comfort Michael and help Sonny cope with the aftermath of the kidnapping. Reese saw how dedicated Sonny was to his children and fell in love with him.
Sonny and Reese's romance was short-lived. They had a falling out after Reese admitted she slept with Sonny's brother, Ric. In a tragic turn, Reese died in the aftermath of a train collision accident.