I suppose I have to disagree with the previous post. Indeed, while I feel she is a minor character, I think that Ismene operates on several essential levels in the play. One reason why I think she is important is that she displays another side to the predicament of being a woman in Greek times. While Antigone is quite committed to breaking Creon's unjust laws, Ismene reminds her sister of what the expectations are for a woman and the importance in following this convention. It is essential that Ismene voices this point of view, as women did have to deal with a different reality (and still do, to a large extent) than the men did. Antigone's dilemma is enhanced because of being a woman, and having to wrestle with the cultural reality that frowns upon independent thought of a woman countering patriarchal laws. Ismene also operates on the level of what happens when passivity and defeatism takes over an individual. She sees herself as powerless to impact change, and her presence reminds the reader of the pain and agony inherent in such a position. If nothing else, the reader is reminded that while challenging situations may plague individuals, there is always action that can be taken and helpless passivity should never be accepted as a state of being in the world.
I think Isemen is very important character in the play.sophocles used her as a tool to bring about the rebelious nature of antigone.Ismene is a passive character while antigone is rebellious.I will give you a very simple example.If you have a painting with a black background and the figure painted is in white so you will see the figure very clearly.Simply Ismene is the black background to antigone.
Ismene is not really that important at all, for she is a minor character. Sorry to disappoint.