The narrator first admires his beloved as "a phantom of delight." She is to him an angelic, otherworldly being. She seems all perfection and hardly human. She appears ephemeral, something insubstantial who won't stay in his life for long: "a moment's ornament." In this incarnation she is a dream figure, a haunting, dancing angel.
Later, she comes to be more real and substantial to her beloved. She becomes a fully solid woman rather than a phantom. She is now
not too bright or good / For human nature's daily food.
In other words, she has gone from being a goddess to a human being to him.
In the third part of the poem, her two sides, otherworldly and human, are brought together into one admired whole. The beloved is both angel and earthly woman. The poet praises her reasonable nature and her noble character, which he finds strong and enduring. He ends by writing of her:
A perfect Woman, nobly planned,
To warn, to comfort, and command;
And yet a Spirit still, and bright
With something of angelic light.
The poet shows the stages the lover goes through, first seeing the beloved as wholly angelic, then as a real person living an everyday life with everyday emotions, and finally, as a being who is both a "perfect" earthly woman and yet shot through with a spiritual light.