The Gray Lady
Reasoning: Unfortunately, we don't know anything about the patron ghost of Ravenclaw except that she is tall. The card represents someone nurturing and generous, practical and trustworthy. So, let's hope she's all of those things. :)
In a traditional tarot deck, this is the Queen of Pentacles.
Notes
As J.K. has so far left this character entirely up to the readers' imagination, I just drew her the way I envision her from her only known name, gray lady, as well as images that came to my mind involving various beheaded English royalty, specifically young princesses, the histories of which I only know vaguely.
This card started off with a lot more stuff in the room: tables with books, a chair, a window, and the tapestry. But when I started refining, and resized the Gray Lady, it just became too busy. I simplified it, keeping only her and the tapestry, which I think works better. Otherwise there would be all sorts of stuff showing through her, and it might get a bit confusing (especially for me!).
The transparency was a great deal of fun - the ghosts are the only cards that have pencil in addition to Copics, to help with that transparent effect.