Jim : Character and personality
Jim's personality and character development are some of the most important things about the "28 Days Later" story. Unlike typical horror/zombie movies, "28 Days Later" focuses on characters and their development, and Jim's is, arguably, the most complex one. The way he goes from a shy boy who blushes when a girl kisses him on the cheek to a rage filled guy on a killing spree makes one of the central points and raises many questions.
In a way, Jim serves as an audience surrogate, because we learn most of the things from his point of view, but he is more than that. His transformation, journey and development are at the heart of the story. It is therefore vital to understand the change he goes through.
Jim starts as a nice and somewhat naive young man. His life previous the accident is never shown, but his character can be observed through his behaviour in the beginning of the story.
His first reaction, after waking up in the hospital, is confusion, which is understandable. One more thing to note is that he wakes up alone and abandoned, which will later be implied to be his worst fear.
While never completely lacking the survival instinct, Jim is shown to be reluctant to follow Selena's advice about killing the Infected "in a heartbeat", or coping with the horrible reality. He doesn't seem capable of taking charge, and depends on others (above all, Selena) and their guidance.
Jim is not a coward, but he is unable to accept the new rules of survival. It is implied he cares about people around him and is easy for him to get emotionally attached. He is also shown to be somewhat shy.
Coping with reality
During the trip to the military base, Jim begins to change, particularly after killing the Infected child. He is still reluctant to accept the truth and act on instinct alone (as advised); however, it is shown he's capable of dealing with the situation, even if he hates to.
Another aspect of his personality is shown during the dream he has. The nightmare are not the Infected, or the Rage virus, but the fear of being abandoned and left alone.
His humanity and emotional attachment are once again demonstrated in his hesitation to kill the infected Frank. He knows he has to, but can't force himself to do it. It is later shown this empathy and emotional attachment are not unique to him: Selena proves herself to be compassionate herself, and nothing like she presented herself to be.
The nature of humanity
Human nature plays the central role in the third act and is, arguably, the main point of the story. Soldiers and their plan is what trigger Jim's rage and his sudden change.
The imprisonment, the horrible "cure" and realization there's no global infection are the things that make Jim take charge. But above all, it's his attachment to the girls. In his attempt to save them, Jim demonstrates incredible power and will. He is no more a naive boy he once was, but a man capable of doing anything, including horrible violence.
Jim's rage towards those who tried to hurt Selena and Hannah, towards those who dared to call themselves "human" is so strong that it makes Selena believe he's infected. But she can't kill him, and the two get a chance to express their feelings for each other.