Harriet is a weakminded but pleasant girl with no past. Emma fancies herself a matchmaker and an excellent judge of character. And she is powerful in her society. A dangerous combination. At first I thought she was Darcyish pre-proposal, but now I think she is far far worse and perhaps even dangerous. She is almost playing god isn't she? She [i]picks[/i] Harriet and then she sets about making a life for her, starting from creating a past to picking her husband. In the process she hurts Robert Martin, a poor unassuming soul. (Smacks of Darcy, but I think the intent is very different).

I think Mr. Knightly half nails it...

[i]"I have not half done about Harriet Smith. I think her the very worst sort of companion that Emma could possibly have. She knows nothing herself, and looks upon Emma as knowing every thing. She is a flatterer in all her ways; and so much the worse, because undesigned. Her ignorance is hourly flattery. How can Emma imagine she has any thing to learn herself, while Harriet is presenting such a delightful inferiority?"(Chapter 4)[/i]
While Harriet is not at all a good companion for Emma, it is even more true that Emma is not a good companion for Harriet. Harriet's simple and weak character requires someone who will not take advantage of her nature, but will appreciate her goodness. Emma in her current conceited state cannot be good for Harriet because her simplicity encourages Emma's manipulation. On the flip side, Harriet feeds Emma's conceit by not challenging any of her notions.