Though Esmé isn't Arthur's first love—or even his first wife, for that matter—she represents his second chance at happiness after all the chilling and terrible things that occurred in his youth:
For some fourteen years now Monk's Piece had been the happiest of homes—Esmé's and mine, and that of her four children by her first marriage, to Captain Ainley. (1.17)
She provides a safe place for Arthur to rebuild his life after all that's happened to him, and we say good for her. With his life at Monk's Piece, Arthur is able to move on—almost—past the death of his wife and child.