Dill’s ulterior motive is to take a peak in order to see Boo. Dill has always been fascinated with Boo because he seems to be a mysterious being. He is someone who never went out of his house which is still considered weird even up to now.
His character is generally known to be good although it may not be obvious in the beginning because he seems to be a rather eccentric character. He made the effort to give little gifts to the other children. He also has some plans in order to save the children from harm. He may not look like the kindest character but he has good inside of him.