Janenna
Of Ninjas and Pirates
Unless maybe she joined the Gaang as a double agent or something, I really don't see how that could work.
I could see that working. She could totally learn all their plans while teaching Aang and then turn on them in the finale.
No, that'd be too predictable. Like, all the fans would be "I TOLD YOU SO" plus, the gaang would never trust her. Maybe the rest would be coerced like liking her like they did zuko, but zuko himself would never accept azula in.