Marcus Flint
Reasoning: Symbolizing someone domineering, overbearing and blunt, this card cares little for the feelings of others. On the plus side, it can stand for someone who is direct, honest and authoritative, who cuts straight to the core of an issue and can be very intelligent. However, it can often manifest itself in rudeness and emotional coldness. I think this fits what little we know of Marcus, except for the intelligence part (as he was held back a year...). However, he is described as having a look of 'trollish cunning'... so I suppose maybe he's smart for a troll, but dumb for a wizard. XD
In a traditional tarot deck, this is the Knight of Swords.
Notes
This card went fairly smoothly. I have a pretty clear image of Marcus, but unfortunately he turned out a bit more handsome than I intended here (I think it's those 'Johnny Depp' cheekbones). This is kind of closer to how I see Oliver Wood than Marcus... but I suppose the Slytherins out there will thank me for not making him hideous. ;)
Upsettingly enough, the pink pen that I was using for his lips splatted ink in a few places, most notably his left hand. I tried to correct this by coloring the spot with a white colored pencil. This actually helped quite a bit, but it's still visible. **sigh** The other splat landed, thankfully, on his darker brown boots so you can't see it. I really need to be more careful when refilling these pens. :B