"Oh. I understand you've had a rather hard time lately. That's too bad. You probably felt you had no choice but to behave outrageously. Like a wild animal. But don't worry, you won't anymore... Because I am going to break you."
— The circus trainer to Appa.
The circus trainer worked for the Fire Nation circus. He was an angry but soft-spoken firebender who came from a prominent Fire Nation family. He was known for his great abilities to train the many dangerous and exotic animals the circus collected and took great pleasure in "breaking" the spirit of all the animals. As a firebender, his specialty was the fire whip, which he used consistently to train the animals. Particular to the point of neuroticism, he was a loner. It is widely believed that he was raised by a sadistic Fire Nation nobleman. He was a rather formidable chess opponent, but a very sore-tempered loser.
He tried but failed to tame Appa to perform in the circus. He refused to feed the flying bison any food, notably cabbages, as punishment for the bison's stubbornness and would continue to do so until Appa did what he was told. However, Appa was able to feed himself using his airbending whenever the trainer had his back turned.
Later, when Appa started to perform in the circus, the circus trainer used his firebending whip to control the sky bison. He was last seen when he was trying to attack Appa as the sky bison was escaping from the circus stage.