She is, but she's not omnipresent, at least not in an active sense. Of all the people we've met in LLtP, she is the most alien to the human mind. She is a force of nature, detached from human concerns and with a world view that might seem confusing or cold. She is aware of this, but makes no effort to change herself.I have a question: why does Erato sound surprised when she sees the Leanan? Isn't she the forest?
Look at her response when the Leanan Sídhe asks her if she's fucked the MC already. Erato merely sees this as a question she naturally has to answer, but the Leanan Sídhe sees it as so much more. These two women could never work well together.
In some ways, you could compare her to Dr Manhattan in Watchmen. He might tell you what you are going to do in 5 seconds, yet act genuinely surprised when you do it.
The Leanan Sídhe is a figure from actual mythology in our own world. While this is my own interpretation of the character, looking her up on Wikipedia might give you an idea of the darker implications of her scene.PS: I really hope I get to see what happenes if the MC accepts Leanan's advances. I get ME2 Morinth vibes here!