I love this description of Robert Preston that Cynthia gives in chapter 43, "Cynthia's Confession":
...for he is cruel in his very soul--tigerish, with his beautiful striped skin and relentless heart.
This captures perfectly his power over Cynthia.
I can't help but wonder about his relationship with Mrs. G--he clearly disappointed her greatly by moving his attentions from her to her daughter while they were still at Ashcombe. Her response to him whenever she sees him, though, has an air of guilt about it that makes me think she allowed him more liberties, to put it delicately, than would have been proper. Just as Cynthia carries the secret of her engagement to him through most of the book, Hyacinth carries the guilt of her prior, perhaps intimate, relationship with him into Hollingford and into her marriage with Mr Gibson.
I noticed that both Miss Browning and Lady Harriet, when facing the fact that Molly was seen meeting privately with Mr. Preston, declare that they will personally go to church and "forbid the banns" to prevent Molly from marrying Mr. P. The mark of a true friend, indeed!