Madam Hooch
Reasoning: The card of impartiality and fairness, and of choosing to adhere to ethics and principals. The Justice card sees all sides of an issue and makes a decision based on the rules without any personal bias. This card holds a great deal of responsibility and accountability, and one must weigh ones decisions carefully.
Madam Hooch is Hogwarts's totally impartial (from what we can see in the books) referee. She knows the laws of Quidditch inside and out, and maintains order and fairness with a stern reliability and without singling out any one house.
The job of Quidditch referee is not to be taken lightly when so many wizards and witches take Quidditch so seriously. In professional leagues, referees must be as fair and honest as possible, not only for ethical reasons, but to avoid being magically stranded in the Sahara by an irate fan.
In a traditional tarot deck, this is Justice.
Notes
Madam Hooch is a fun one to draw. Short, spiky hair is tricky for me, but I think I found a style for her that I like. I was concerned about the background for this one. It went through several different incarnations, one of which included Harry and Draco on either side of her. But I couldn't figure out what to do with the rest of the background, and in the end I decided it would be better to have all four house colors represented. After all, there's more than just Gryffindor and Slytherin competing for that Quidditch Cup!
When I started getting into the color tests (which I did roughly on the computer) I was afraid that the background might clash horribly, or that she would get lost in it. But once I took a deep breath and just colored it, I think that it works OK: the colors are varied enough not to clash, and Madam Hooch's black & white striped referee outfit stands out uncolored, unbiased, among them. :B