The most interesting thing about Rosedale is the way Lily's vision of him – and accordingly our own vision – changes throughout the novel. Start at the beginning, when Lily encounters Simon outside of the Benedick:
He was a plump rosy man of the blond Jewish type, with smart London clothes fitting him like upholstery, and small sidelong eyes which gave him the air of appraising people as if they were bric-a-brac. […] Mr. Simon Rosedale was a man who made it his business to know everything about every one, whose idea of showing himself to be at home in society was to display an inconvenient familiarity with the habits of those with whom he wished to be thought intimate. (1.1.132, 1.2.3)
Our first picture of Rosedale is that of a social climber with the not-so noble ambition of being a member of the social elite. That really seems to be his sole purpose in life (other than making massive amounts of money, which apparently gives him no trouble at all).
At first, as readers, we tend to judge him for this. He seems shallow, self-serving, and generally a small and petty character. But, as the novel continues and the world around Lily is revealed to be morally corroded, selfish, and even downright cruel, the small kindnesses that Rosedale offers Lily single him out as a kind and caring man – a far cry from the gossip we first meet on the stairs of the Benedick. And Rosedale makes a decent case for himself and his social ambitions in the second half of the novel:
Why should I mind saying I want to get into society? A man ain't ashamed to say he wants to own a racing stable or a picture gallery. Well, a taste for society's just another kind of hobby. Perhaps I want to get even with some of the people who cold-shouldered me last year – put it that way if it sounds better. Anyhow, I want to have the run of the best houses; and I'm getting it too, little by little. (2.7.49)
Compared to Bertha's machinations and Grace's treatment of her cousin Lily, wanting to hob-nob with the elite doesn't sound so bad. And Lily realizes this, too. By Book II, Chapter Five, "she no longer absolutely despised him" (2.5.38). Er, right – Lily isn't exactly welcoming Rosedale with open arms, even after she decides she wants to marry him. But that's just more social determination at work. Remember that Lily is programmed to dislike men like Rosedale, just as much as Rosedale is programmed to want a woman like Lily and the world that she could offer him as a wife.