Katara didn't ask if he was sure. She wasn't going to give him a chance to second-guess his decision. She was touched that he was willing to listen to her enough to give the mask up, and she was awed that it was her he'd chosen to give it to, but she didn't react to either of those emotions. She simply nodded once and stepped closer, taking the mask from his hands and holding it tightly in one of her own.
"You're doing the right thing, Zuko," she finally spoke quietly, once the mask was firmly in her grasp and out of his. Offering him a small smile, Katara added, "I think your uncle would be proud of you."
Not that she knew Iroh very well, if at all, but it seemed like the right thing to say.
Of course, this still didn't change the fact that she knew something wasn't quite right about Zuko. He was struggling with something and she wasn't sure what it was, or how to help him. If she even could help him, that is.
"If there's anything else I can do..." She trailed off and sighed, her shoulders hunching slightly as she looked back to the mask. Her brow furrowed, lips pressing into a thin line. Then she faced Zuko once more.
"The right thing isn't always the easiest," she stated, voice little more than a murmur. "But I know you can do this, Zuko. I... trust you, almost as much as I trust Aang and Sokka and Toph. I hope that someday you can trust us, too." She wasn't speaking about the mask, now. She was speaking in general, or more specifically about his sister, Mai, and Ty Lee's sudden arrival. Katara assumed he would realize that much himself, though. Some things didn't need to be spoken out loud.