Lady Macbeth, Macbeth's ambitious wife. In fact, it could be reasonably asserted that she is far more culpable than her husband in causing the play's tragic events to unfold. In her quest for a crown she knows isn't rightfully hers, it is she not Macbeth, she manipulates her weaker husband; who is at least initially inclined to reject the idea of killing Duncan until his wife, who berates him and questions both his masculinity and courage.
A central idea in the play is what makes a person strong or, for lack of a better word, tough? As the action of the play unfolds it turns out of be strength of a moral code rather than of body or will that counts for the most.