After that kissing incident between Kayano and Nagisa, Karma didn't stop teasing his friend about it.
He usually said things like 'Did you use the situation as an excuse to kiss her?' or 'You know nobody would think about kissing a person only to make them stop raging. Except couples.'
When he saw Nagisa, he often greeted him with 'Here comes the prince!'

Nagisa slowly started to get tired of it. It really annoyed him - especially because if he hadn't done what he had, Koro-sensei might have not been able to bring Kayano back. It needed doing and he didn't think about it in a romantic way.

So after a few days, he immediately snapped at Karma's first insult:
"You should stop saying nonsenses, even Nakamura-san gave it up days ago!"
Rio raised her head and and said with an evil grin: "Could it be that you're jealous?"
And waited for Karma's reaction.

The red haired boy got surprised, but a moment later he wore his usual sarcastically calm expression again.

Instead of him, Nagisa replied:
"W-why do you think so? He has never shown signs of liking Kayano that way..."
"I'm not talking about Kaede-chan" said Rio still grinning and forming "Na-gi-sa" with her lips. She waited for the boy to realize what she had meant.
It didn't take too long. A few moments later Nagisa's whole face flushed.
"Y-you know I-I'm a guy!"
"And you know a guy can be gay."
"But he's not...!" then he stopped. How could he say it instead of Karma himself? So he added: "Right, Karma-kun...?"
But the asked boy didn't say a word.

_

After class ended, Nagisa went to Karma to go home together.

"What are you going to do at home?" he asked cheerfully, making Karma look at him and feel relieved.
The red haired boy had expected Nagisa to interrogate him about the conversation earlier. But he soon tensed up again as Nagisa continued. "I hope you'll stop teasing me from now on. Although I don't know why you're enjoying it that much."
"Didn't you find Nakamura's theory reasonable?" said Karma with a sarcastic smile.
"As you said, that's just a theory. I want to hear it from Karma-kun himself."
"Eh, are you interested? ~" Nagisa decisively nodded. "OK, I'll tell you but don't get mad~" he seemingly enjoyed making Nagisa frustrated. And the game still didn't end. He was preparing for the final blow - the most Karma-ish way to get out of this awkward situation.
"I wanted to see how Kayano reacts to those comments -"
Nagisa interrupted him:
"So you really do like her-"
"Nope~ I just thought she was feeling uncomfortable."

Nagisa started to get more and more annoyed. Although he had asked Karma to stop teasing him, it seemed like it was only fuel to the fire.
"I've already said sorry to her."
Karma's evil grin made him even angrier. Although usually Nagisa was patient, this had been going on for days and he had enough.

"But that doesn't change the fact how bad that kiss must've felt... since it was from you."
For Nagisa, this was the last straw.
He grabbed his insulter and pulled him behind the tree they had been walking next to.
"I'll show you I'm not a bad kisser!"
"Nagisa-kun, what are you-"
Then he pushed Karma's back to the tree and silenced him with his lips.
Karma left his mouth open in surprise so Nagisa's tongue easily slipped in. He began to count the hits as he was exploring every corner.
1 hit, 2 hits, 3 hits... 10 hits...
After 35 he lost count.

Karma was just standing pushed to the tree and didn't move an inch. He felt his legs weaken and his heart burst.
His mind got so blank that he even forgot it was the result of his masterplan...

They both started panting rapidly as they parted.
Nagisa's anger had disappeared long ago replaced by pleasure and now he realized what he'd done.
"W-what did I just..." He covered his mouth with his hand, his whole face blushing.
Karma's face was as red as his hair while trying to get back to his usual self.

After a minute of staring at each other, they both managed to calm down a bit.
Nagisa broke the silence:
"Will... will you take back what you said?" he asked, still blushing.
Karma grinned again.
"I couldn't decide, so could you do it again?"
The red color spread to Nagisa's ears.
The only thing he could say was: "So Nakamura was right after all."
All he could do in embarrassment is to run away and leave the surprised Karma.

From that day, Karma never mentioned that kiss with Kayano again.