Ron on the Chess Knight
Reasoning: The Chariot is the card of masterful control, a strong will towards achieving victory or a goal. It is a card of having confidence in your abilities and winning by applying disciplined effort. Ron is the best chess player we've been introduced to in the series, and uses those skills unique to him to win the enchanted chess game guarding the Sorcerer's Stone. He takes control of this situation and moves forward with grim determination and does what it takes to emerge victorious.
In a traditional tarot deck, this is the Chariot.
Notes
This was a very old sketch that I revisited. I loved the way Ron looked in the sketch, but I am less happy with how the finished card turned out, which is sometimes the case with sketches. Oh well! Still like Ron himself, but not the background so much. *shrug*