Toph is the first one to know, because of course she is. Just because she's blind doesn't mean she can't see, after all. She doesn't know how that little detail can slip her friends' minds, but it does on a regular basis. Not that Toph is complaining. It gives her an advantage, because it means she picks up on things way sooner than the others.
Like Zuko and Katara. Toph knew something was different between those two years ago, after their little field trip, even if neither of them acted on it until more recently.
She'd noticed the change in Zuko way back on Ember Island. The way his heart would pick up its pace in his chest when Katara walked in the room (or even when he talked about her or, Toph likes to think, even when he'd just think about her sometimes), or how his body heat would rise when she got close to him (was that happening because he was trying to keep her warm, or was his body doing its own thing?), or the way the lilt of his voice would change just so when he talked to or about Katara.
Katara surprisingly kept her emotions in check better, and Toph actually didn't notice a single sign of how she felt about Zuko until after the end of the war. Sure, if any of them were healers, they'd undoubtedly would have stayed by Zuko's bedside and nursed him back to health too, but Katara was different when she came out of that room.
Suddenly, Katara was reacting to Zuko the way he'd been responding to her for the last few months. Her heart would beat like a drum in her chest and Toph could feel the way her lungs would constrict when she saw him, as if she couldn't quite draw a full breath. Her voice would become higher, more melodic, when she talked to him. Toph could even smell the sweet perfume she'd wear when she knew she was going to see him.
It was so obvious, that Toph almost can't believe that no one else could see what was right in front of them. Zuko's casual rejection of Mai when she got out of prison (which he told everyone was just because they'd grown apart, but Toph knew he was lying) and Katara's rebuff of Aang ("We're better off as friends," Katara had told everyone, which was true, but it wasn't the only reason) should've made it known to everyone then. But it didn't.
Toph has never brought it up in so many words to either Zuko or Katara, but she's tested it out in small ways over the years. Like when she'd ask Zuko to describe how Katara looked one time, and he'd spent a good three minutes rambling about how soft her hair looked or how pretty her dress was, and he just went on and on about the color of her eyes. Or when she'd asked Katara's opinion on what Toph should get Zuko for his birthday, and Katara had described (in detail) all of Zuko's favorite things.
But oh no. Those two didn't like each other. Not at all. But Toph knew the truth, even if no one else realized it yet, the two of them were goners.
Suki is the second one to figure it out. It takes her a few years longer than Toph (but hey, Toph has her seismic sense, and her other senses aren't too shabby, either), but Suki realizes it not long after Sokka proposes to her.
She and Katara were talking about the wedding, mapping out all of the details, and from there the conversation had divulged. Suki knew that Aang's affections were one-sided, but it had been a few years, and Katara hadn't seen anyone else in any serious capacity. So Suki had asked her if there was anyone.
Katara's cheeks had grown red and she'd looked away with a mumbled "No", and that's when Suki knew. It clearly wasn't Aang, and who else could it be? Katara had jumped at the opportunity to work with Zuko in the palace. She'd been there for nearly three years.
What Suki didn't understand was that if Katara liked Zuko, why didn't she act on it? But since Katara never came out and told Suki directly how she felt about Zuko, Suki didn't pry, deciding instead to let her best friend come to her when she was good and ready.
And spirits, did it seem to take forever for Katara to come clean about her feelings.
Sokka notices it next. The truth is that he's suspected it for years, for almost as long as Toph actually. Maybe people think he's an idiot when it comes to love, but Sokka understands better than most people give him credit for.
He would see the way that Zuko looked at his sister back on Ember Island. And he recognized that look, because Sokka knew that he looked at Suki that way, too.
And, after a while, he'd catch Katara looking at Zuko the same way Suki looked at him.
And really, Sokka wouldn't have been mad if Zuko and his sister got together. Zuko is a good man, and has been for a long time. Ever since he joined their team and helped end the war, he's only proven himself more and more. He knows that Zuko is honorable, and kind, and strong. (Not to mention that he'd give his life for her.) He'd be good for Katara, and she would be good for him.
So when Katara tells him that Zuko has offered her a position as an ambassador, he tells her to take it. She's uncertain about leaving him and Dad and Gran-Gran, so Sokka tells her that she has put everyone before herself for too long. It's time for her to be selfish, to do something for her.
She had boarded a ship the next morning, and her letters only grew happier with time.
Sokka has been waiting for that one letter to come, the one where she tells him he should be expecting a new brother-in-law, but it doesn't happen .
Sokka knows that Katara is anything but a coward, so he wonders why she's biting her tongue. It's so obvious that Zuko feels the same way. If only one of those two hard-headed, stubborn idiots would just open their mouths.
So Sokka just keeps waiting.
Aang is the last one to figure it out. And maybe he should've realized it sooner (like, way sooner), but in his defense, he was sort of preoccupied with taking out the Fire Lord and ending the war, and then trying to establish peace in an unstable world. He had Avatar duties. He had to focus on rebuilding his culture.
And maybe he should've picked up on it when Katara told him that they were better off friends. He was hurt when it happened. He'd loved her ever since he'd first laid eyes on her, and he really thought by the end of the war, that she'd realize she felt the same way about him. But that moment never came.
In fact, she'd told him Aang, I love you...but not like that. I think we're better off as friends. She'd crushed him, just like that, and Aang had wracked his brain trying to figure out why. But he'd had to put his heartache on the back burner to deal with more pressing matters, and then he just...tried not to think about it.
Or really, he supposes, he didn't want to think about it.
Because thinking about his best friend loving the girl of his dreams hurt more than any words Katara could say to him.
So yeah, Aang had been the last one to know. He's mostly okay with that.
Toph is the first one to break the silence. She tips the rest of her fire whiskey down her throat, slams the decanter down on the table, and grins so wide it nearly looks painful, her milky eyes almost seeming to look at Zuko and Katara.
"It's about dang time," she tells them. "I've been waiting for one of you to grow a pair for years!"
"Years?" Katara squeaks.
Suki points her thumb at her chest. "Same for me. I figured it out when you would get all moon-eyed when me and Sokka were planning our wedding."
Zuko looks at Katara, a soft smile curving his lips. "You'd get moon-eyed? Were you thinking about our wedding?"
"No," Katara insists.
"Liar," Toph snickers.
Katara ignores her and looks at her brother. "Did you figure it out too? How long have you suspected?"
"Oh...you know." Sokka shrugs, but he can't fight his grin. "I noticed it back on Ember Island."
Both Katara and Zuko's eyes grow wide before Katara looks at Zuko. "Wow, I guess the only ones we kept our feelings a secret from were each other."
He takes her hand and brings it to his lips, pressing a kiss to her knuckles. "I guess so. "
Toph leans back against her seat and kicks her feet up on the table as she tucks her hands behind her head. "Well, I guess I do have an unfair advantage. But man, was I getting impatient."
"I was getting ready to play a little matchmaker," Suki admits with a sheepish grin. "I mean, I'm sure half of the palace suspects it. The two of you were so obvious back then, I bet you're way worse living together!"
"Oh, they definitely are," Sokka quipped. "You could practically hear the sappiness in Katara's voice through her letters over the years."
"Is that so?" Zuko chuckles as he smiles at Katara.
Katara blushes and smiles back, before finally looking at Aang, who has yet to say anything. She offers him a kind smile, and he returns it weakly.
"I'm happy for you guys," Aang said, although Toph didn't have to tell everyone that he was lying, at least a little bit.
"Thank you, Aang," Katara tells him, hoping to convey with her eyes everything she's feeling: gratitude, sincerity. "We were...hoping you'd officiate the wedding. Alongside the Fire Sages, of course."
"To mark the beginning of a new era, two nations coming together," Zuko adds diplomatically.
"I would be happy to," Aang says, and his smile is a little more relaxed this time.
Sokka claps his hands and whoops loudly. "Whoo-hoo! Royal wedding! This is going to be awesome! My sister is going to be a queen!"
The friends melt into laughter. Maybe Zuko and Katara were really the last ones to know, but none of that matters now. They all know now, and their friends are happy for them. Zuko and Katara are happy.
It's more than enough for them.