I love everything about this! I'm pretty sure that J.K. stated that his patronus was a wolf, but he hated it so he always preferred to produce a non-corporeal patronus (which is the kind that you drew, where it doesn't assume a particular form). Very well done!
I love how you drew Lupin. But I can't fault your dementor. The flowing, tattered cloak seems so real. The detail is brilliant. I don't know whether you intended it, but I actually noticed a small skull shape near Harry's sleeve, which I really liked.