Sir Nicholas de Mimsy Porpington (Nearly-Headless-Nick)
Reasoning: This card symbolizes someone attractive, friendly, vigorous and encouraging. Well dressed and regal, Nick always has a friendly word for Gryffindor students (as well as often helping Harry avoid trouble). An impressive crowd of guests showed up for his 500th Death Day party, so apparently Nick is quite popular.
The one major way Sir Nicholas differs from this card is that this card typically represents one who is not easily upset and who takes defeat with grace and good nature. Granted, Nick does try to accept the rejection by the Headless Hunt gracefully, but his efforts come across as rather transparent (haha...) .
In a traditional tarot deck, this is the Queen of Wands.
Notes
J.K. Rowling recently released a sketch she did of Sir Nicholas, and I tried to follow her guidelines and still keep him my own interpretation. This actually wasn't too hard, as my previous drawings of Nick were pretty similar to this. The only thing that changed was the hair.
This card was fun, as it is always a challenge to draw someone looking so serious and noble in such silly looking period clothing. :).