Sokka
After family, Sokka believes most strongly in reason and logic. And so despite his initial distrust of Zuko, he's willing to act logically based on the facts. The facts are:
1. Sokka has a sword.
2. Zuko has a sword.
3. They both have some time on their hands when Zuko's not teaching Aang.
4. Battle could always be imminent and they need to keep their edge.
Enough said.
Nobody except Katara is surprised when they start sparring every afternoon. Everybody but Katara is glad that soon after Zuko actually joins them at the fire sometimes instead of hanging uncertainly in doorways.