Emma is really not a "character" in the traditional sense of the word. She’s actually more of a concept – she is the subject of Stephen’s adoration, but we have no real sense of her as a person. She is the inspiration for some of Stephen’s first experiments in verse; as a child, he writes a super-romanticized poem to her, and ten years later, still infatuated, he writes her a villanelle (a complicated, super-style-conscious poetic form). Neither of these poems focuses on her specific characteristics. Instead, they vaguely evoke Stephen’s feelings of love, desire, and confusion.
Like a lot of cases of puppy love, Stephen seems to be more enamored by the idea of capital-L-Love than with Emma herself; he is devoted to the poetic ideal of a "beloved" to dedicate his poems to. Inspired by the ideal women he encounters in the poems of Byron and in The Count of Monte Cristo, Stephen creates an image of Emma that he idolizes (for ten years!). This interest in following a certain poetic model is yet another indication of Stephen’s gradual but inevitable transformation into an artist.
At the end of the novel, we finally start to see Emma as a flesh-and-blood creature; her encounters with Stephen reveal her to be nothing more than a normal Irish girl, rather than a divine and unattainable creature. All the same, Stephen cannot bring himself to start a relationship with her. After their last meeting, Stephen seems shocked to note that he "likes" her – all along, he has idolized her image, but we get the feeling that he knows almost as little about her as we do. When Stephen, on the brink of his departure from Ireland, mentions that he’s seeking a certain "reality of experience" out in the world, it’s an indication that some of the past experiences we’ve witnessed have lacked reality – like his relationship with Emma.