Likely the most important, if you are trying to actually rank them, would be Janie Johnson, then you could move to her parents since the three of them together drive most of the action of the story. You might also be able to put Reeve shields up there but it is difficult to decide whether or not he would surpass the parents in order of importance.
Again this is sort of a difficult question as you could interpret it in different ways. My feelings go for Janie, then Mom and Dad though.
two of the main characters are Janie Johnson, she is the girl who thinks her face is on the milk carton. Reeve, Janie's boyfriend helps her find her true mother, gives her rides and does kissy kissy with her a lot.. hope this is useful!