Saying that Korra was surprised to learn that Asami was a waterbender would be an understatement—she hadn't suspected it at all!
As for why they had to keep it secret, she had no idea, but it was obvious that both Jin and Asami were serious about it, so she had no choice but to play along until they told her why.
As a result, Korra now spent a lot of time visiting both Asami's and Jin's homes.
While the two girls deepened their friendship and worked on helping Asami master waterbending, Jin felt it was finally time to deal with the traitor and with Yangchen.
There was no avoiding it—he had spent too much time near Korra, and Kyoshi's presence couldn't be hidden from the current Avatar forever, especially if he wanted to help her awaken her spiritual side early so she could start airbending.
"I see," Kyoshi said, reviewing the plan to deal with both Avatars. "So, we'll break the cage that imprisoned me to lure them in, then move them to that place in the Spirit World while they're still shocked, and finally… we'll do that."
"That's the plan," Jin nodded. "I've already exchanged information with the spirit, and he's more than happy to help. Anything you'd like to add?"
"Inkasha will have to stay behind," Kyoshi said firmly.
"Aw, fine…" Inkasha crossed her arms, pouting. She wanted to teach those bad Avatars a lesson too.
"Could you keep an eye on Korra and Asami for me?" Jin asked, preferring to give her something less dangerous—and less potentially fatal—to do.
"Alright, Korra's been eating my oatmeal cookies anyway," the spirit replied reluctantly, though it sounded more like a personal grievance.
"Ready?" Kyoshi asked.
"Let's go. We have to move fast."
…
Somewhere in the Spirit World.
Yangchen opened her eyes and rose to her feet. With a single step, she was beside Aang's spirit, who also stood up at her arrival.
"The cage is broken. Something happened to Kyoshi," Yangchen said.
"This shouldn't have happened," Aang frowned. "It's a shame Korra's spiritual sense is so terrible—we can't get much information from her side," he sighed.
"No one can match the spiritual sense of Air Nomads," Yangchen said matter-of-factly, her tone full of arrogance. "Come on, we have to deal with her before things get worse."
Aang sighed again, his expression complicated.
"How did we even get to this point?"
"You know how."
Aang said nothing.
Of course he knew. Didn't Yangchen realize it was a rhetorical question?
"Let's go together. We should strike while she's still weak from breaking the seal."
"…Fine."
Aang and Yangchen moved through the Spirit World and appeared before Kyoshi's prison, but—
"Something's wrong. Where is she?" Yangchen looked around, frowning.
"Yangchen, this doesn't look like an escape," Aang pointed to the seal. "Something broke it from the outside. Could it be a dark spirit?"
If a dark spirit had fed on Kyoshi, it would be a major threat to the mortal world.
Suddenly, a massive surge of chi grabbed them both and dragged them elsewhere. Caught completely off guard, their faces froze in shock. When they were finally released, they recognized the place—and horror filled them.
"Too late~," came the sound of wood being pecked as something crawled closer. "Those are wonderful expressions—perfect for my collection!"
"NO!" both cried at once.
A series of wet, unpleasant sounds echoed within the hollow of the great tree as Koh "collected" his prizes. Once the Faces Stealer had claimed both of their visages, Kyoshi and Jin entered to retrieve the now faceless spirits of Aang and Yangchen.
"Ah~, a pleasure doing business with you," said Koh, clearly delighted with his new trophies. "Don't hesitate to call again if you have more… jobs like this. I'll give you a good deal next time too."
"Understood," Jin replied with the same indifferent, neutral expression as Kyoshi—unlike Yangchen and Aang, whose vivid looks of shock and confusion had just become Koh's new treasures.
"What exquisite irony~," the spirit laughed, retreating into his nest as he realized they both knew his limitations. He watched with glee as they took away the faceless, emotionless spirits of the two former Avatars.
Koh, the Face Stealer—a spirit almost as ancient as the Moon and Ocean (Tui and La)—was obsessed with collecting the faces of every being, spirit, animal, or human, for they were the masterpieces of his mother, the Mother of Faces.
Yes, it was a kind of twisted parental love.
But his thefts had a strict rule: his victims had to show an emotion in their face for him to steal it. Joy, disgust, surprise, hatred—anything would do.
On the other hand, anyone who kept a blank, emotionless expression was completely safe from his power.
In fact, Aang had nearly lost his face twice when he first met Koh. Once because the breath from the face Koh was wearing was so foul he almost couldn't keep still; and a second time when he smiled for a moment upon getting the information he needed—but he was spared because Koh had his back turned, and Aang had managed to regain his composure before the spirit turned around.
Kyoshi and Jin had exploited those very conditions to ambush Yangchen and Aang in Koh's lair. Since they weren't prepared to remain expressionless, Koh had no trouble stealing their faces.
"Hurry, let's finish this before the other Avatars realize something's wrong," Kyoshi urged.
They stripped the two of their Air Nomad robes and dressed them in the plain clothes of Fire Nation commoners from Aang's era, concealing their true identities even further. Then, without either spirit resisting, they threw them into the Fog of Lost Souls.
Korra would still be able to draw upon their strength if she tried, but no matter how much she called for Aang or Yangchen, she would never receive a response—or find them again.
As for the change of clothes, Kyoshi thought it unnecessary, but Jin knew that the so-called "prison" of the Spirit World wasn't as inescapable as legends claimed. After all, Tenzin had managed to break through it and rescue his family.
So Jin made sure that even if something like that happened again in the future, no one would ever recognize those two Avatars.
As they walked away, there were no questions like "Did we go too far?"
Kyoshi and Jin had taken their revenge—without harming Korra or destroying Aang and Yangchen's spirits. Now Kyoshi could "reconnect" if she wanted, and Jin no longer had to worry about the schemes of a jealous Avatar.
As for the rest of the Avatars, unless Kyoshi chose to tell them, none would ever know what had happened to the missing ones.
"I think we deserve a little celebration for a job well done," Kyoshi said with a smile, taking Jin's hand and giving him a playful, suggestive look. Then both vanished, heading somewhere private for their intense celebration.