Dreams walk in her mind whenever she isn't looking. He likes to watch her, keep an eye on her. He knows how dangerous she can be.
"I... I... don't know, Professor," she stammers when he asks her questions in class, so he no longer asks her. He is afraid of the day when she begins to question her sanity, to question the world of imaginary shadows in which she lives. When she slips from dreams to delirium - that will be the final fall.
The first one when Tom Riddle left a hole in her mind, opened to dreams. And madness.
Her hair is crazy-wildfire red with little sun kissed streaks running through it, her brown eyes wide and... not vacant, just not at attention. She is bright, he has noticed that in his years of watching, but distracted. It was only three years after the incident that they found out the cause.
At least once weekly McGonagall is in a panic in the staff room - that Weasley girl has taken one of her nocturnal "walks" by the lake again. No known charm or spell or ward can keep her inside the castle; Filch and Mrs. Norris have never caught her. They only know by the muddy footprints that lead up to Gryffindor Tower's dormitory for Seventh-Year girls. And to her bed.
Severus envies the girl her freedom, and, to be truthful, her slipping sanity. At times he wishes he could make his peace with reason and be done with all of that.
One day he joins her on her nightly escape from Hogwarts, her nightly pilgrimage to the lake -
"An altar to Selene?" he asks her.
She looks up; surprised, but not startled. "She is beautiful, isn't she?" she says eagerly. "She's the most lovely of them all. You, of all of them, would understand. Does she talk to you as well?"
"No. I am afraid I have not yet had that honor." He looks to the moon, full, white, and pure against an indigo silk sky. "Do you speak often with her?"
"No, Selene spends her time with those far more worthy than I! But sometimes she visits. I think she will be there tonight. You must meet her!" She extends a hand, shyly; hesitantly, he accepts it. "She will like you," she tells him firmly.
They walk to the lake hand-in-hand, where the moon shines in all her glory, reflected in the silver waters of the lake.
Dream kisses Delirium, in search of release from logic and truth; and their mortal counterparts echo their touch. Delirium was once Delight, after all - is all not right in the world? On this night?