Hmm.... yeah, favored environment fits. Although Sokka never seems to be more or less effective depending on his environment, but I could be wrong. He does however have an amazing amount of knowledge about the fire nation, so favored enemy COULD be in play there, or just a very high knowledge skill. Perhaps switch out wild empathy for something? Which is, I assume what you did with animal companion? Switched it out for another ability, in that cast Favored Enemy (Fire Nation Military)?