Sorry for the long wait (in case anyone is interested), here's the third part of the story.
I hope you didn't expect anything big from Nagisa to say.
It's karaoke time - sorry for my weird choices, I'll explain them at the end of this part.

Karma had wanted to play around with Nagisa. Now he got it.
The blue haired boy joined the game.

"Well, it's just... I had no clue it was too much for you to see me dressed as a girl."

Karma hadn't expected Nagisa to say something like that. Recognizing his purpose and going against it. 'Haha, I've known Nagisa's really something!'

He stood up and went to the monitor.
"Alright then, time to sing."
"W-why so sudden?"

Nagisa immediately found the answer out when Karma started to sing.

The lyrics of the song was completely about romance, kind of provoking, probably meant to embarrass him.
But it didn't really work: all Nagisa was paying attention to was Karma's voice that suited the song.
He realized the lyrics only at the last lines:
"Yasashikute atsukute hikyou na kisu de..."*
("With your tender, passionate, yet cowardly kiss...")

"Well?" asked Karma after he finished singing.
Nagisa's eyes were sparkling and face a bit red.
"It was great, your voice is beautiful! You could sing more!"

Hearing it made Karma both surprised and delighted, then he got slightly disappointed for not having reached the expected effect on Nagisa.
"First, I want to hear you!"
Nagisa shaked his head.
"But I don't..."
"No excuses, I'll even help you choose."

Karma looked in the song list. It didn't take more than five minutes, he pointed at one.
"You know this one."
Nagisa gulped. "Yeah... I do." Then he wondered: "Did you choose it to prevent me from singing one that would make you embarrassed?"

Karma smirked. "Nope, that'll be the next. I just simply want to hear you sing this one."

Nagisa took a deep breath and grabbed the microfon.

...
"Hello, shooting star, hello, shooting star again
Matteita yo..."
("I was waiting for you")*

His performance left Karma amazed. Nagisa's soft, high-piched voice was perfect for this kinda slow and melancholic song.
He waited for Nagisa to finish, then went to him and pulled him in his arms.

He whispered: "I thought this would be a good choice... But I didn't expect it to be this amazing."

Nagisa froze.
"K-Karma... T-thanks..." He soon calmed down and closed his eyes, wrapping his arms around Karma's upper body. "And I didn't expect you to do this."

Then he realized what he was doing and pushed the red haired boy away.
'I got carried away and almost forgot it's probably just like a game for him...! '
"What's wrong?" asked Karma.
"Enough of playing around."
"I wasn't..." started Karma, but he stopped. "Then sing me one more."
"It's your turn." A playful smile appeared on Nagisa's lips. "How about me choosing for you ?"
Karma smiled back:
"Alright, the choice is yours."

Nagisa didn't have to think too hard about it. He found a song that he could imagine being sung by Karma.

"Shoudou alarm (impulse alarm)*", that was innocent and absolutely perfect for his beautiful voice. Nagisa couldn't help but stood there without moving an inch, totally captivated by the performance.
This song reached its purpose, instead of Karma's first choice: it made Nagisa's face much redder than the previous passionate song.

Seeing the blue haired boy's reaction made him really satisfied.
"Oh, did you like it that much?"
"N-no.. Erm... I-I did like it, but that's not..."
Karma leaned closer.
"Not what?"
Nagisa turned his head away and said:
"It's nothing... The next song is mine!"

Karma sighed.
"Don't change the topic... But alright, for the last time. And to give you an opportunity to beat me, I'll help you choose an embarrassing one." grinned Karma.
"I already know what 'help' means to you..."
But Nagisa actually was relieved that he had managed talk about something else.

"Owaranai bouken (An endless adventure)"*, literally. At least it could've been compared to what Karma was feeling while listening to Nagisa.
He took a deep breath and went to the blue haired boy.

"Now I really want you to be mine."

Nagisa freaked out:
"Wha- WHAT?"
"You heard me."

He tried to calm himself down, thinking it was more like a battle that will be won by the one who makes the other surrender.
More or less successfully, he responded:
"You're really into teasing the other to death."

Karma grinned:
"It's not teasing that I'm into. What I'm into is Na..."
Nagisa cut him off, face deep red:
"Honestly, that's too clichéd...!"

Karma leaned uncomfortably close to Nagisa's face.
"Oh, do you know what I wanted to say?" The grin grew even wider. "Or do you want me to say that I'm into you?"

Nagisa had really, really wanted to say something to have a fling at the red haired boy, but he acted before thinking.
"Just shut up." he hissed and helped Karma to do so by sealing the boy's lips with his.

It was too late to retreat - and this time, Nagisa had been the one to start it.
To ease his guilt, the excuse crossed his mind in a second: 'I just wanted to win this stupid teasing "contest". Yeah, that's right. It's the same as an Plus, it's his fault for saying those weird things.'

He opened his eyes to see Karma's reaction. Surprised orbs looked back for a brief moment, then they disappeared under his eyelids.
Encouraged by this response, Nagisa slid his tounge in between those parted, inviting lips.

He felt Karma's hands get ahold of his hair, in response, Nagisa wrapped his arms around Karma's neck.
The kiss was so passionate they both started panting rapidly after separating.

They stared at each other with slightly red faces.

'What do I say now...?' panicked Nagisa while trying to catch his breath.

He immediately found out that his theory was correct: he had really wanted to kiss Karma again, because it was somehow different from all he had had previously.
So then, what to say as an excuse?

Karma helped him by breaking the silence:
"Geez, I lost. You completely seduced me."
Well, it wasn't help at all. Actually, that comment made it even worse.

Nagisa's slightly blushing face got deep red.
"I-it's your fault...!" he yelled.

Karma grinned.
"Actually, it was you who was flirting with me the whole time."

"You started it...!"
"But you continued."
"You kissed me first...!"
"And you kissed me second."
"I'm doing it for the third time if you don't shut up!"
"Then I'll go on talking."

At that time, Nagisa's face was already red to his ears.
"Argh, stop saying these weird things!"
"Alright, then I'll shut up and act silently."
Karma grabbed the blue haired boy's wrists, pushed him to the wall and captured his lips.

Nagisa wanted to push Karma away, but he lost all strenght in his arms.
'If he's just playing around, it'll hurt. I need to stop it.'
'Dammit, it's too...'
He moaned as Karma's tongue slid inside his mouth.
'Haah, just a bit more...'

He gathered all his strenght, put his palms on Karma's chest and pushed him backwards. He finally managed to break the kiss and to make Karma take one step away from him.

"I- I'll give up, forget about this stupid teasing-game!" he yelled, turning his head away.

"Nagisa, I gave in earlier. It has nothing to do with the 'game' anymore."

Nagisa's eyes widened.
"Then... You've been serious...?"
"Yup. All along." Seeing the blue haired boy raise an eyebrow, he corrected himself: "Well... Actually after hearing a few words of your conversation with Terasaka, I came only because I wanted to have fun watching your acting. But then when I saw you -"

"You..." Nagisa interrupted him. "...you knew it...! ...and still let me act as some weird girl...!"
"Hey, I also have the right to be mad. It's really degrading of you to think I wouldn't recognize you." He chuckled. "Serously, not even wearing a wig nor any make-up, still trying to hide your identity..."

Nagisa hissed.
"It's not my fault that Terasaka-kun didn't even get me one..."
"Well... He actually did me a favour, I didn't need to waste time by taking it off your head."
"W-why would've you...?"
Karma smiled:
"Because I wanted to see Nagisa in girly clothes..." He took a blue lock between two fingers. "...with Nagisa's beautiful blue hair."
Before the smaller boy could say anything, Karma continued, switching back to the former question:
"So yeah, I came here yesterday with the intention of playing around with you. It must've been after seeing you wear those clothes that I started to purposely do all these things. I invited you to this date with the intention of confessing to you."

Nagisa's brain was processing what he had heard.
"Karma, are you really...?"

Karma took a deep breath and asked:
"Nagisa, will you go out with me?"
"Karma, you know I'm not a girl."

"How wouldn't I know... I've seen your upper body several times. It's true I haven't seen that, but I know what's there and I'm not disgusted by it at all." Karma then grinned. "Or shall I check it out now?"
Seeing Nagisa's annoyed look on his deep red face, he added: "Just kidding, just kidding." The grin got kind of evil when he said: "Actually, I can perfectly see the bulge in your pants."
His comment earned a punch under his chin.
"At least don't say it...!" hissed Nagisa.

Karma rubbed his chin.
"Ouch. Sorry, I'm serious now." He looked into Nagisa's sky blue irises. "I'll ask it in another way: Will you be my boyfriend?"

Nagisa's widened eyes slowly went back to normal and he replied with a smile on his face: "I will."

*: Songs: 1. Monochrome no Kiss (Kuroshitsuji opening 1-2) - because it has a passionate lyrics, perfect for Karma's little game and I could imagine Okamoto Nobuhiko to sing it.
2. Hello, shooting star (I'm sure you know this one - Assassination Classroom ending 1) - so sorry for choosing one which is actually from the original story (for me it's not realistic at all), but I really wanted to include it as one of Nagisa's song, because it has a version actually sung by his seiyuu, Fuchigami Mai. Hearing it made me fall in love with this song. If you haven't heard it, you must!
3. Shoudou Alarm (Sekaiichi Hatsukoi - Kisa's character song) - because it's actually sung by Okamoto Nobuhiko, Karma's seiyuu. That's why it amazed Nagisa more, since this one was really written for Karma's voice and he sings it in a much softer tone. If you think it's Karma who's singing while listening to it, it's wonderfully strange (there's no way he'd sing like this xD)
4. Owaranai bouken (Kyou Kara Maou! - Wolfram's character song) - because it's about love (literally Yuuri, the MC) and also sung by a female seiyuu playing the role of a male character (Saiga Mitsuki - she's amazing!)

So yeah, sorry again, I hope it makes sense.

BTW, this was the last part.
Or almost the last, since I'm currently writing an epilogue. We still need to find out what Terasaka's going to do now (there'll be a bit more).