Luna Lovegood
Reasoning: This card is utterly perfect for Luna. It symbolizes the unconscious, the unknown, the mysterious. It tells you to stay calm and passive, to tap into your imagination and look beneath the surface. Luna acknowledges things that cannot be proven, that exist beyond the rational world. She is strong in her passivity, not allowing trivial things like stolen possessions or name-calling to upset her, and understanding that not even death should be cause for despair.
In a traditional tarot deck, this is the High Priestess.
Notes
I had a great - yet challenging - time with this card. It was one of the first I sketched up, not even originally intending it to be a card. I knew very specifically that I wanted the sky magenta-colored, which ended up causing a huge delay as the pen I needed was on backorder for ages.
The thestral's face was a great deal of fun, but it's body was a nightmare. I vacillated between portraying the body facing head-on, or to the side....but which side? And how to attach the wings (which underwent many variations themselves...) to the body? Argh! Eventually, after much sketching, I got it to a point that I liked.
Then I started coloring it, and became afraid that her Ravenclaw robes would clash horribly with the pink-magenta sky. And at first they did, but then I added some purpley tones to the sky and I think it works well. It would be silly if her outfit perfectly matched the sky, anyway. :p I also experimented with putting buildings in the background (the silhouette of London when they go to the Ministry), or the forest, but in the end I decided that would be too much DARK against the DARK thestral and kept it simple with the sky.