Well, considering Jeff Greene is the main character in this book, this is a most appropriate question. What is ironic about the way you asked the question is that Jeff Greene is also the only narrator, so we see everything through his eyes. We don't have the opportunity to see what other characters might think of him except in his own words.
The main character of Jeff Greene is insecure even in his body language and his physical appearance; his shoulders are hunched and he always projects a worried look on his face. At seventeen years old, Jeff comes home one day to find a note from his mom saying that she left the family. She also warns Jeff not to bother his father about much of anything. Because of this, Jeff is unable to grieve about his situation. Being extremely sensitive, Jeff has trouble dealing with most of the things in his life at this point. Further, Jeff's love of nature and music as well as his interactions with other characters in the book help him to become more mature as he grows up. By the end of the book however, Jeff is able to take risks, and we know that he will be able to trust again.
In conclusion, it's important to note that the author only reveals Jeff's character through his own words because he is the narrator in the entirety of the plot.