Poor Rosanna. She's not a major character, but she plays an important role in moving the story forward. Like Rachel, if Rosanna had been more forthright, the mystery would not have been as long and drawn out. If Rosanna had gone to Lady Verinder with the stained nightgown, or even to Franklin himself, instead of hiding it in the quicksand, the mystery would have been solved. But Rosanna's reticence, or unwillingness to speak out, isn't the same as Rachel's. She refuses to speak because she's in love with Franklin Blake.
This is actually kind of scandalous: a female servant in love with someone from the upper class? Especially a female servant with a disability? It was practically unheard of during the nineteenth century for any woman to admit that she was in love with a man before he made advances first. It was considered improper. And for a servant girl to admit that she was in love with a social superior? That was just socially wrong.
Or was it? Collins makes Rosanna a very sympathetic character. She doesn't have a lot going for her – she doesn't have a pretty face, one of her shoulders is deformed, she used to be a thief, and the other servants don't like her. But Collins shows that it's totally natural that she should fall in love with Franklin Blake. She has as much right to fall in love as Rachel has, even though Rosanna has less chance of happiness.