Strength
Reasoning: The Strength card represents more a kind of quiet, inner fortitude rather than a flashy, feirce in-your-face kind of strength. It is more about character, an unshakeable spirit and compassionate endurance. It subdues by being gentle and persistant rather than by violent force.
Prongs is Harry's Patronus, the personal guardian and protector of the wizard or witch who casts it. The Patronus can only be formed by tapping into your deepest, happiest feelings and thus defeats evil through the power of love rather than sinking to the same evil depths. It takes great strength to call up such feelings when confronted with those soul-sucking dementors and other such threats!
In a traditional tarot deck, this is the Star.
Notes
This card took ages. While it's very similar to the old version of this card I did several years ago (Harry is practically the same), I experimented with lots of different variations before finally returning to something closer to the original concept.
These experiments included having Harry actually casting the Patronus and Prongs taking out a herd of Dementors. Both of those looked too cluttered to me... I wanted PRONGS glowing and the focus of attention. I even considered leaving Harry out of it, but it's kind of pointless to have a Patronus trotting through the forest with no one to protect. Besides, the strength of the caster is the entire point, so it was only fitting to have Harry there, the origin and benefactor of Prongs's power.
I'm still learning how to do convincing nighttime coloring, but I do feel this is one of my best efforts at it to date.