Lady Macbeth is the penultimate person in the play, especially at the beginning, next to Macbeth himself. And though she does not survive to the end, her influence on Macbeth lasts throughout. .
She is most influential person in Macbeth's downfall, next to the witches. However, her relationship with him goes far deeper then that of the witches. It is my belief that the witches act only as a trigger to start the events in the play, and that Lady Macbeth herself was the driving force behind Macbeth's actions. It is she who he contacts when he meets the witches, and immediately trusts her with the prophecy he is given. .
The relationship between Macbeth and his wife is paramount to the understanding of a major theme of this play. At first it would appear to be an equal partnership. However, I believe Lady Macbeth was the dominant of the two character; she could have persuaded Macbeth to do anything if she so wished. And though she does not openly exercise her power over him in public, in private she often uses humiliation and emotional bribery to manipulate Macbeth to execute her will.
The first scene in which we see Lady Macbeth is Act 1 Scene 5, in the first half of which she is reading the letter sent by Macbeth about his meeting with the witches, and about half way through Macbeth, himself enters, having caught up with the messenger who delivered the letter. Immediately we see the nature of her relationship with Macbeth, and have a strong sense of her character. The first thing that you notice of course, is that Lady Macbeth is reading a letter that must have been written mere hours after the events contained happened. It is a letter from Macbeth, containing potentially treasonous information about his meeting with the witches, now, the actual meeting itself could not be considered treason, because that could not be helped, but for Macbeth to even hope for the witches prophecy to become truth would have been very dangerous ground.