Rouch concept: His father was once a young Northern Water Tribe captain who at some point went rogue with his men to get back at the Fire Nation. They landed in a coastal region of the Earth Kingdom, to aid the locals and establish a resistance, and there the PC's parents met each other.
Since a young age he was trained to become a formidable waterbender and join the ranks of the resistance, which over the years became however more and more daring, until one day the Fire Nation heavily retaliated. While his father stayed back to fight, he found himself forced to flee with his mother, to finally settle at a later point in the starting village.

I envision him as a resourceful fighter who learned to make the best of whatever little water is available in whatever form, ice, vapor, moisture, sweat, and even the blood of the enemies in favorable circumstances. From a mechanical point of view, a Prodigy who mastered bloodbending, the idea (if feasible, of course) is that he would use Steady Stance to Empower himself somewhat reliably and then strike with Freeze Blood.