A/N:
This is a little snippet that came to my mind watching an MMD with Mephisto and Amaimon on youtube ('Bad Apple' is the song) and I wrote it down so I wouldn't forget it. Also, Mephisto is totally a person that would do something like this.
I do not make any profit from this and neither Mephisto nor Amaimon, or that video belong to me. This is purely for my entertainment, and hopefully yours as well.

"...And that would be all. You understand?" Mephisto asked after the video finished playing.

"I'm not sure I do, Aniue. Why do we have to do this again?"

Mephisto's eyebrow twitched in annoyance.

"Now listen here, little brother. It was YOU, who endlessly pestered ME to spend some time with you, because you were bored. And since you are obviously not able to entertain yourself without wreaking havoc, and I decided to generously sacrifice some of my time for this, out of my busy schedule..."

...That mainly consisted of catching up with the newest season of anime (really, between this, his Academy and the Illuminati there was hardly any time left for himself - good that he was apt at manipulating others into doing some of his work for him).
But there was no reason to mention that.

"... so~ it's only natural that we would be doing something I want to do, is it not?"

Amaimon stared at him, then back at the screen.

"It is, Aniue. But..."

Another twitch.

"Yes...?"

"Why dancing?" Amaimon finally voiced his reason for hesitation, carefully. Mephisto was scary when angered, after all. "I have never done something like..."

He trailed off at the sight of the devilish smile that spread on his brother's face.
At least he wasn't angry. But that kind of smile was no good news either.

"Ah, yes, yes. Of course you haven't. But you won't need to worry about that, it is really easy."

There were his concerns, brushed aside. But it wasn't over.

"... Besides, you did watch closely, didn't you? You remember the choreography."

It was something a demon learned with age - to remember moves on sight and make them their own. It was an important skill for fighting, and nobody would survive a thousand years in Gehenna without it. Not that he had to use it that often, anymore.

"..." Amaimon's silence was answer enough, and soon he found himself dragged along by a chipper Mephisto to an relatively empty pocket dimension of his.

Looking around the room, Amaimon was astonished by the level of preparation Mephisto seemed to have put in this. There was a big sound system, the lighting was perfectly tuned and the stage decorated. At the moment Mephisto was humming, setting up a camera, presumably to capture their effort on film.
Something clicked in his mind.

"Say, Aniue..."

"What is it?"

Amaimon had the feeling his brother was daring him to go back on their little arrangement.

"If you wanted to do this that badly, why haven't you done so already?"

A short pause.

"Because it is a choreography for two people, silly. I can't just do this on my own whenever I want."

"Then, couldn't you have asked someone else?" That question was treading into dangerous territory, but now he was curious.

Anyone else would probably have missed the slight change in atmosphere, but Amaimon could tell his brother was close to snapping again.

"Don't worry, I won't." Amaimon answered the silent question, if he was going to have another try at worming himself out of this one. At that, his brother relaxed, and he was able to breathe once more.

There were no people in the Order Mephisto could rope into dancing to the song of a vocaloid with him. Everyone was too stuck up. Besides, what was the fun in just ordering someone to dance at his side, if they were all stiff. Now, his brother on the other hand, would do nicely. His movements were raw but had a certain beauty to them.

"If I did that, I wouldn't be able to spend time like this with you, little brother, now would I?"

This answer actually surprised Amaimon - for all his brother's complaining about him, this was a side he would not get to see very often. But every now and then, Mephisto would show that he did, in fact, care for his family. Even if he would not admit it to himself.
But Amaimon knew better than to tease him about it.

When Amaimon moved to take the stage, his brother's voice sounded out once more.

"And I get the girl's part~!" (1)

"Yes, Aniue."

(1) This is, a reference to when I first saw the animated choreography in a different video. Twins, boy and girl. The role Mephisto is taking is that of the girl.