Edmund has a lot of things in common with a Greek guy named Pygmalion. In the myth, Pygmalion was a sculptor who made a piece representing his ideal woman and then fell in love with her. In a sense, Edmund, too, created his own ideal woman – Fanny – and eventually chose her over the real, complicated, and imperfect Mary. Speaking to Fanny, Edmund notes, "I am glad you saw it all as I did." The narrator adds, "Having formed her mind and gained her affections, he had a good chance of her thinking like him" (7.11-12).
Edmund is the stalwart moral character of the book – he disapproves of the play, he lectures people, he becomes a clergyman, and he's often judgmental. And yet, Edmund also acts in the play the Crawfords and Bertrams set up. His love for flawed Mary shows signs of changing him – for better or for worse, depending on your opinion – which Fanny notices to her dismay. Edmund opens himself up to someone new and different and sincerely wants to marry her. But Edmund never fully seems to be willing or able to change himself in order to compromise with Mary. Rather, he expects her to do all the changing:
The issue of all depended on one question. Did she love him well enough to forego what had used to be essential points – did she love him well enough to make them no longer essential? (26.11)
Edmund expects Mary to change her ways for him and to conform to his idea of what is moral and right. This tension comes to a head when Mary fails to take Maria and Henry's affair as seriously as he does. He is upset that she sees the affair as simply "folly," that she expresses "no horror, […] no modest loathings" (47.18) at Maria and Henry's behavior. In fact, she's far less upset by their behavior than by the fact that they let themselves get caught. So Edmund ends their relationship. He doesn't manage to make it work with the women he really seems to love.
What's interesting is that we get very limited details about Edmund's relationship with the woman he eventually marries: Fanny. Edmund acts like an affectionate older brother to Fanny until the very end of the book, where the narrator gives us a brief paragraph about how Edmund fell in love with Fanny eventually. We hear a lot about Fanny's love for Edmund, but we don't get much of the reverse at all. And even Edmund's relationship with Mary, a much more sizeable chunk of the book, is very lacking in details. We see them together almost exclusively through Fanny's eyes, or else hear about them briefly from the narrator: "They had talked – and they had been silent – he had reasoned – she had ridiculed – and they had parted at last with mutual vexation" (28.21).
Rather than hearing the full conversation, we just get a brief description of it. In a way, Edmund spends much of the novel like a square peg in a round hole. He seems to be living in two separate, parallel stories. In one story, he's the male lead who's involved in a turbulent love story with Mary. In the other story, he's supposed to be the romantic interest of our protagonist, Fanny. At times it seems that we get more interaction between Fanny and Edmund not just because the book is Fanny's story but also because that's the story that Edmund "ought to be" participating in. What complicates this further is that Edmund's romance with Mary doesn't seem like a mistake. They both seem to genuinely love each other but, in the end, their relationship comes to a very messy end. Who do you think is a better fit for Edmund: Mary or Fanny?