"You were with her?" was Katara's first reply. She didn't even think about it. She just said it. Out of all the people in all of the Fire Nation that she could imagine Zuko with, the girl who threw the knives was not one of them. "But, she's so... blah!" Realizing she was standing there, badmouthing his former girlfriend, she at least had the grace to flush slightly and look away.
"Sorry. That was rude," she muttered. It was clear from her tone she was only apologizing because it was the right thing to do, not because she was really worried about having insulted Mai.
Studying a spot on the wall for a moment, Katara thought over what he'd said and finally returned her attention to him. "If you're going to try to be good, and work on a team, you have to break that bad habit of yours," she stated evenly. "People can't trust you if you're always doing whatever you want to do without any thought to how it affects them." It was the best advice she could think to offer, considering part of her still doubted his sincerity in all of this.
Another heartbeat passed and she sighed. "Okay," she finally agreed, "I'm... sorry I said some of the things I said to you. I guess you aren't that horrible anymore." That was the most she was willing to offer. She still didn't trust him, and she still wasn't sure if he was telling the truth about things, but she was willing to give him the benefit of the doubt.
If he turned out to be lying, though, she was going to kill him. Literally. And nobody, not even Angel and Cordelia, would be able to stop her.