From the story one thing is clear, and it is that, up until Leslie's arrival, Jesse was a one-dimensional character.
Due to his mother's favoritism towards the sisters, and the lack of time spent with him by his parents, he was an insecure child with loads of repressed talents, such as his art. Because of the lack of interest he instilled in his family, he was also an angry and lonely child with a tendency for frustration, and and introvertive personality. He was yet to find himself, his true voice, and open the door to his imagination.
As we know, all of this changes when Leslie comes along and becomes his best friend. His personality after that becomes then three-dimensional. You can sense the liberation that he experiences once his art and talent is out, quenching all the anger from frustration that he experienced before. He is yet again changed in the end after losing Leslie, but he is an even more aware and complete character. On and all, Jesse is a character that ungergoes a whole transformation from beginning to end.