Samuel Coleridge
Coleridge himself is the narrator of the poem. In his own voice, he carries out a conversation with Sara, his wife-to-be, telling her of his great anticipation of their marriage and future life together. He reveals his connection to nature and relationship with God using poetical metaphors.
Sara Coleridge
Sara was Coleridge's fiancee while he was writing the poem. She is featured in the poem having a conversation with him. Though she is the addressee, she is not the main character of the poem. Coleridge, instead, talks to nature in the form of the Aeolian Harp.
God
God is the unifying force to Coleridge's musings. He credits God with all of his blessings and all of creation itself. In rather intriguing language, Coleridge concludes that God is felt in the soul of everyone simultaneously and all of nature.
Lute/Harp
While not an animate character, the harp (or lute) plays an important role in the poem. As Coleridge is describing his impending marriage autobiographically, he chooses the harp to symbolize poetry. The harp then stands for the very words which he is writing in how they relate in his consciousness to everything else. He is describing how he relates to his fiancee, nature, God, and the very words on the page before him.
