Since the first commenter did such a beautiful job of addressing the first part of your question, I will address the last part. A round character is a complex character, one that cannot easily be summed up in a sentence or two. Both Elsa and Helen are round characters. Elsa, on one hand, is supremely alive to Helen's inspiration and artistry, and she's built a friendship with this older woman because she was so impressed by Helen's spiritual freedom and unwillingness to have her role dictated to her by society after her husband's death. On the other hand, Elsa is, at times, mean and quite harsh when she speaks to Helen, even studying the older woman with "cruel detachment." She is kind enough to care about Miss Helen and drive twelve hours to spend one night with the her when Helen is in great pain, but she is unwilling to listen to Helen's explanation of why she lied about her accident. Elsa is, at once, kind and cruel, as well as a bit jaded. She is sad that her romantic relationship with a married man has ended, that she trusted and lost; she is angry at the world and herself because she is alone—especially because she terminated a pregnancy and now seems to regret it. At the same time, she seems to believe in the possibility of learning to love and to trust again. Her pessimism is tempered by optimism.
Helen is a round character because she, too, is complex. She seems, at times, especially in her conversation with Elsa, to be meek and somewhat submissive. However, she is also revealed to possess a great deal of strength in that she's chosen independence rather than social acceptance. She has chosen to pursue artistic creation as a way of inspiring and fulfilling herself rather than continue to attend religious services that felt, to her, like a "terrible, terrible lie." Helen cannot even bring herself to say "no" to Marius when he tries to force her into a home for old folks, but then she becomes more and more articulate when explaining to him the importance and purpose of her Mecca. She has been incredibly courageous in her decision to depart from social convention and instead do what makes her feel free. Ultimately, she chooses to be alone, even though Marius loves her, because it is really the only way to retain this freedom.