Forrest met Jenny on the school bus, on his first day of school, and she invited him to sit with her, when nobody would, and she took an interest in him, by asking about his legs. The two soon became best friends, spent a lot of time with each other, and particularly played by a large tree.
Jenny and Forrest remain best friends through high school, but when Jenny attended college, the two were separated, because Forrest couldn't go to that school because it was an all-girl college. One night, when Forrest went to visit Jenny in the pouring rain with a box of chocolates, he noticed a car in which Jenny and her date, Billy, were trying to make out, but when Jenny accidentally hit her head against the car window, Forrest mistakenly believed that Billy was hurting her, opened the door to the car, and repeatedly punched him. Billy got mad and drove off. Jenny, despite being annoyed, invited Forrest into her room and it was implied that the two had sex.
Jenny eventually went through a rough time in her life, when she got expelled from her school, and got a job at a strip club. It was unknown what happens to Jenny for the next few years, but she eventually became a hippie. She was reunited with Forrest in Washington DC, while he made a speech at the National Mall, and they spent the day together, and went to a Black Panther Party safe house. However, Jenny's boyfriend, Wesley, the president of the SDS at Berkeley, hits her, and Forrest tackled and punched him. They were then forced to leave. The following day, Jenny left with some fellow hippies, but not before Forrest gave her the Medal of Honor that he won in Vietnam. After that, Jenny succumbed to a life of drugs, and even attempted suicide.
One day Jenny showed up at Forrest's home and stayed with him for a while. After they lived together for a while, Forrest asked Jenny to marry him, but she refused, and said: "You don't want to marry me." Later that night, Jenny came into Forrest's room, said she did love him, and the two had sex. Jenny left early the next morning, and left Forrest heart-broken and lonely.
Forrest eventually found Jenny, a few years later, and she had a son. Jenny told Forrest that her son's name was Forrest, like his father. Although Forrest didn't get it right away, he eventually realized that he's his child. Jenny later revealed that she was ill and that she was suffering from a unknown virus with no cure (supposedly HIV). Forrest asked Jenny to come and live with him, where he promised to take care of her and little Forrest. The two married soon after, in a ceremony at Forrest's house, and Jenny died six months later.
At the end of the film, Forrest talked to Jenny's grave, which was located by the tree that they played on when they were young. Forrest told her how much he and his son missed her.