Somewhere along the way, Shakespeare developed a focus on a mysterious "dark lady" who appears in Sonnets 127–154. Whether she reflects an actual person Shakespeare knew or merely some ideal he conjured up, we learn some interesting details about her through descriptions in various sonnets.
In Sonnet 130, for example, Shakespeare contrasts the ideals of typical feminine beauty with the imagery of this "dark lady." Her eyes are not so bright. Her lips are kind of pale. And most women would not be flattered to have a man tell them that "black wires grow on [your] head." Going even further, Shakespeare even describes this woman as having breath that "reeks." However, at the end, he maintains that this love is rare and true. In effect, he is explaining that he is not blinded by love; he sees this woman for what she is and cherishes all of her—even the imperfect parts.
This "dark lady" possesses dark qualities both in physical appearance and in temperament. In Sonnet 127, Shakespeare uses this description: "My mistress' eyes are raven black." And, of course, we know that "black wires grow on her head" from Sonnet 130. But additionally, Shakespeare makes this comment in Sonnet 132: "Thine eyes I love, and they, as pitying me, / Knowing thy heart torments me in pain." In Sonnet 137, Shakespeare writes, "When my love swears that she is made of truth, / I do believe her though I know she lies."
Whether this lady was a real person in Shakespeare's life or not, it is clear that she represents a complexity that seemed to draw him nearer to her. She represents something dark and intangible, tormenting his sense of reason.
People have written entire books on this topic and have been captivated by this mystery since the Victorian era. The link below provides some information about who this lady might have been (if, in fact, she was a tangible person).