She gave him a look that clearly said she couldn't believe he'd just said that. How, she wondered, could he be so blind? With a slight shake of her head, she sighed softly. Falling silent for a moment, her gaze dropping to the ground, Katara fell into a contemplation. There had to be some way to make him see reason. She needed a way to make a point without using him as the example.
Finally, she spoke. "When I first met Aang," she admitted, slowly raising her gaze back to the prince, "he didn't tell us he was the Avatar. Later on, when we found out, he said it was because he didn't want to be the Avatar. It took him a long time to realize that who he is as Aang - all of the doubts he has and all of the mistakes he makes - are the same as who he is as the Avatar. Without those doubts, he'd be reckless. Without those mistakes, he would never learn."
Twisting on the bench so she could face him a little better, she added gently, "This, I don't think, is very different. You are the Blue Spirit, Zuko." Oh but that sounded strange to say out loud. She continued, though, more determined than ever to make him understand.
"Who you are, with and without that mask, is the same. The mask itself doesn't change anything. All it does is give you a reason to hide... and you shouldn't be hiding."