This is actually a very astute question to ask, because the way that the characters are balanced against one another actually imitates one of the chief conflicts that Jane Eyre herself undergoes, as she battles between being overpowered by feeling without judgement and judgement without feeling. The way that the main characters are balanced against each other according to these two extremes therefore indicates the kind of internal conflict that Jane herself faces.
Consider for one moment how St. John Rivers and Rochester are used as two extremes in this way. Firstly, the way that fire and passion are associated with Rochester is balanced by the symbolism of ice and snow that is attached to St. John Rivers. Both present opposite extremes of the kind of conflict that Jane herself is experiencing. To take a lesser example, consider how Georgiana and Eliza are described as Jane returns to Gateshead to visit her dying aunt:
True, generous feeling is made small account of by some; but here were two natures rendered, the one intolerably acrid, the other despicably savourless for the want of it. Feeling without judgement is a washy draught indeed; but judgement untempered by feeling is too bitter and husky a morsel for human deglutition.
The way that the characters are used as foils for each other in this wonderful novel therefore points towards the internal conflict that is raging within Jane herself, and the majority of the characters can be seen as representing one or other of these extremes.