In any other story, this character would have been a villain without a doubt. Most likely she would have been an invader who would have to be defeated by the (probably male) protagonist, but here that's not really what we get. Yes, this woman makes a regular habit of luring innocent men to their deaths, but in a weird, twisted sort of way she's also the most human character in the movie.
We see the whole world as she does, and to be fair to her Earth is a very unfamiliar environment. For lack of a better world, it's completely alien to her and we see that in a variety of forms. One thing that comes up a lot is her interactions with humans. Most of the humans speak with distinct accents, as you'd expect for a film set in Scotland, but the accents used are extraordinarily thick.
I've heard my share of accents from watching British movies and television programs, but the accents used by half the men in this film are almost incomprehensible. By contrast the heroine speaks with a somewhat more subdued accent. It sounds vaguely English but not really to a point where you can identify a specific dialect. This actually makes a lot of sense as it helps to emphasize just how unaccustomed she is to speaking the language, she knows what she means to say but she doesn't entirely understand the men she is talking to.
We see a bit of this in one scene where she stands on a beach. She just watches, with an apparent mix of curiosity and confusion. She only talks to someone when addressed directly, but then she sees a married couple in trouble, both getting caught up in some big waves. She pulls the man ashore only to bash a rock over his head and drag him off, apparently failing to hear the cries of his child (or at the very least, not understanding that it was his child or why it made those sounds).
Later on she hears a radio report of the man's body being found and his family's disappearance on the radio but there is something in the way she reacts. She doesn't necessarily show remorse, but she's not exactly cold about it either. It's more like she is just confused. Perhaps she just doesn't understand how seriously people on Earth take this kind of thing. It all ties in with the idea that her mind is completely alien and we couldn't begin to understand how it functions.
In her interactions with society we do see that she finds much of it to be extremely confusing. She sits down in a restaurant and apparently orders cake right away (as opposed to a main course) and she struggles to figure out how to use something as simple as a fork. Later on when staying with a friendly young man she watches a comedy program on television. The man bursts into laughter constantly but she doesn't seem to fully understand the concept of humor.
Of course, this character does have some strangely human qualities. After all in the film she has the most depth out of anybody. Most of the men are fairly minor parts, few if any even have names. We don't find out much about them other than bits and pieces of personal relationships. We do see that she is capable of love. One sub-plot concerns her meeting up with a young man suffering from what I've been told is a condition called Neurofibromatosis that causes facial deformation.
She actually manages to sympathize with him in his loneliness and allows him to feel her. In fact she feels so bad for him that she even spares his life (though unfortunately the guy was murdered by someone else shortly after). There are also the scenes where she interacts with a potential boyfriend. She stays at his house and the two do make love. She does seem to be grateful but eventually she feels the need to leave.
In the end, she meets a rather tragic fate, one that ends up being quite a tear jerker for a character who at first hardly seems sympathetic. It is a curious role we have here. Scarlett Johansson does a perfect job capturing the essence of an alien who is... well... alien. She has a mind we cannot begin to fathom, and at the same time Earth is as puzzling to her as she is to us.