Although the female characters in Shakespeare's plays are marginal, how are they crucial for the story to unfold?
For example, Lady Macbeth, Margaret or Lady Anne in Richard III, Queen Gertrude, Ophelia in Hamlet.
While the plays rarely focus on the female characters, Shakespeare makes their presence essential to the story as a whole. The story of Macbeth's rise to power starts not with with his wife, but with the witches he meets who claim to foresee him as King. This plants the seeds of the idea in his head and feeds his ambition. It is Lady Macbeth who then pushes her husband further. She encourages him to kill King Duncan and stands by his side when he is then crowned King. Her eventual mental breakdown from her guilt, and her suicide, profoundly affect Macbeth and change his perspective on all he has "accomplished."
Hamlet's first major trouble in the play comes not from his father's murder (which he doesn't know about) but his mother's quick remarriage to King Hamlet's brother, Hamlet's uncle, Claudius. Hamlet is absolutely repulsed by the decision and thinks the marriage is incestuous. His faith in women and in humanity is shaken by his mother's actions. Unfortunately, he takes some of this out on his true love, Ophelia. He treats her very harshly in an effort to appear crazy and thus find out the truth of his father's murder, but in the midst of his ranting he says some very rude things about the fickleness and shallowness of women. Hamlet wants to save his mother from this marriage, but he ends up at the end of sharp sword due to his accidental murder of Polonius and his part in pushing Ophelia to true craziness and suicide.