One literary device, which is more often associated as a philosophical device, is the dialectic. This involves the discussion of a concept and its opposite. Another way of thinking of it is that dialectic involves a contradiction (X and its opposite) and a possible solution, balance, or synthesis (combination of X and its opposite). The speaker in the poem describes his ideal woman as a perfect balance of opposites: "And all that's best of dark and bright / Meet in her aspect and her eyes." She is described as a perfect balance of dark and light (physically, and perhaps describing her personality as well - her "aspect" and later, her face expresses serene thoughts). It is a delicate balance. "One shade the more, one ray the less," and she would be half as beautiful.
Byron uses imagery to describe this woman. The imagery is used in simile and metaphor. The imagery is interesting because Byron lets the images speak for themselves. The images describe the woman's face and how she walks but these images are also applicable to her sense of herself. In the last stanza, he uses adjectives more often used to describe personality: calm, eloquent, goodness, peace, and innocent.
This last focus on the goodness and innocence of the woman (combined with the other images of physical beauty) instill that same idea of balance. Here, instead of darkness and light, the balance is mind and body.