The secondary characters in Antigone are collateral damage in the war between Creon and Antigone. These two people are single-minded about being right and on doing what they think is right. (Their family connection is so obvious in this area!) During this battle of wills, the people who love them most suffer the most, and I do feel some sympathy for them.
I realize that there are many ways to see the side characters, but I tend to see them as victims of the standoff between Creon and Antigone. Certainly, both of the main characters are worthy of some praise because they stand for their beliefs without wavering. Yet, there is some level of selfishness in their actions, and I think that the side characters demonstrate this. They are the casualties of the standoff between the two main characters. Ismene is one such character. She recognizes the need for Antigone to stand for her principles, even willing to sacrifice herself for the sister. Yet, she is treated with scorn by Antigone, almost being reduced to being called an apologist for evil. Another example would be to consider Haemon. He is the son of Creon, indicating that he has to possess some degree of loyalty to his father. He is also in love with Antigone, already suggesting a degree of loyalty to her. He is in a no win position, indicating to me that Haemon is probably one of the best representations of the ancient definition of tragedy. This conception consists of pitting characters between two equally desirable, but ultimately incompatible courses of action. Haemon is here, located in a condition where happiness is impossible. Ismene and Haemon are side characters of the play who are held as emotional hostages of the main characters.