Jim is quick to think and quicker to act. He does not pause to think things through like Will does but rather goes with his gut feeling every time. Jim's mother cares for him very much but borders on being overprotective, even though Jim is anything but afraid of the world. He values his freedom above all else, and he vows never to have anything that can hurt him. Jim, unlike Will, is not constantly empathic. Sometimes he shows grave concern for others but at other times he can be almost oblivious to the dangers that surround those he cares about while he pursues his own ends. Jim's incredible tunnel vision makes him a great friend for Will, but in the case of the carnival it nearly provides drastic results. Once Jim has his sights set upon something he will not let it go, and so even though both Will and Mr. Halloway explain several times that the carousel is nothing but a cheap trick, Jim still desires it. He understands what they say when they explain that using it will alienate you from your friends, family, and possibly even yourself, but Jim also does not let anyone close enough to him for that to be a problem. If you cannot be hurt, then there is no harm in riding the carousel, except for the possibility that you become Mr. Dark's partner for eternity.
But fortunately for Jim other people are looking out for his well-being, even if he is not. Will and Mr. Halloway save Jim's life in the end of the book and when he comes to Jim seems to have changed slightly. He seems to truly care about his friends and is able to reason out for himself why the carousel is bad. The entire book in a way is Jim's journey to become comfortable enough with himself to let someone else in. Early on in the book he is willing to sacrifice Will in order to get on the carousel, but by the end Jim seems to know who is on his side and who is not.