Disclaimer: Dragon Ball inspired this origin story as they have good villains:

* We can all agree that Equestria Girls doesn't have enough villains or good ones. This is a story about a young adult named Artcraft. He is a tan human with dark hair, hazel-green eyes, and nerdy glasses. He's about as shy as they come; even Fluttershy would be like a stalwart knight by juxtaposition. This naturally made him ideal for bullies to pick on relentlessly; he only hung around Sunset and her friends as they were the only ones who didn't try to bully him. He respected them for that reason and found them likable.

During his private time, he would converse with his imaginary friend, Buu. Being a mental construct that was created from the grief that dwelled within Artcraft, he nodded and listened intently to him; Artcraft wasn't aware that this was shaping Buu's personality to have a hatred of all humans. He was, essentially, creating the world's most dangerous villain off of his venting, and he never knew it. Artcraft only thought about the despair he felt at that time.

Everything came to a head when Artcraft tagged along with Sunset and her friends; they investigated a falling anomaly that crashed into the Earth's surface. It was a glowing pink and completely intact chocolate bar. They didn't know that it was part of Discord's shenanigans before he was reformed; an unknown time skip had occurred, and this happened before Discord's first imprisonment by Celestia and Luna. Discord had decided to create relics containing fractions of his chaotic magic in his infinite boredom and then tossed them into the multiverse. Because that's totally not going to backfire at any point, right?

Artcraft, unfortunately, had a weakness for chocolate; he detailed the bar he ate as "the greatest chocolate bar he ever had in his life." This chocolate bar had 25% of Discord's maximum power in it for centuries until it reached Earth. The transformation took hold immediately, making his imaginary friend manifest and take over as the raging Artcraft-Buu; when he absorbed the Mayor of Canterlot City to regain his body after getting blasted by Sunset's beam of friendship, the mayor sadly lost his life, but this had... unexpected side effects for Artcraft-Buu, he became a fat and jolly pink guy with an innocent mind and the insane power of Discord's magic.

Since Artcraft-Buu was turned into what he was because of chocolate, he wanted to turn others into candy, cookies, and chocolate. He made genuine friends with Buff Biceps, who is the Equestria Girls' version of Hercule Satan. After nearly losing the only friend he ever had, who was also teaching him that killing was wrong, Artcraft-Buu was, understandably, distraught since a bandit ran in there and shot Buff in his chest. After healing Buff from his mortal chest wound, he struggled to keep this evil form in check and then purged it out, so there were two Buus: the fat one and the skinny grey one. The innocent was named Artcraft-Buu, and the evil was named Majin Buu.

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This doesn't scream a good villain either to be honest. Majin Buu worked because even when he was goofy, he still showed his sadistic side just often enough to remind us that this is a villain.

Compared to some of the villains and monsters, this would fit right in with Equestria Girls and is miles better than a few I can think of.

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What does that have to do with Asperger ?

It was made by someone who has it.

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This, isn't a very good idea.

Here is why.

Majin Buu is show to be a severe threat, said to be capable of eradicating a galaxy single handedly. This places everyone in extreme danger with no possible defense.

Sunset and their girls could barely wreck a building with their magic combined, if that. They just aren't capable of standing up to threat like Majin Buu, not especially in his true form.

You need to take into account the vast difference in demonstrated power between the partied involved before trying to move forward with an idea.

Otherwise, it will just fall apart.

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In the universe of Equestria Girls, this version of Majin Buu is different from Dragon Ball; this one has power scaled by the Chaos Magic he wields. All the way down to how he was created. Keep in mind that this version of Majin Buu was initially a figment of Artcraft's imagination before it took hold and controlled him.

So, yes, this Majin Buu will pale compared to the original one in Dragon Ball. The only reason he has his power is because of Discord. Who later regrets what he did, though it'll be too late at that point. Also, Sunset and her friends can merge together into a being called Harmonious. This helps them even the playing field with more powerful foes.

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I don't know, it feels too much like you're playing with fire to me.

Also, rehashing Buu's arc pretty much beat for beat feels a little "undercooked", you might want to consider some variation in places. Otherwise it's might not go over well.

You will also need to explain Harmonious at least somewhat. If you can't reasonably justify it, you shouldn't use it.

Don't take the "it's magic I don't got to explain crap" approach. That will fall flat at best.

It's also just plain better for your world building if do provide at least something of an explanation.

Harmonious is a temporary transformation; all elements must be present, or it will not work. Sunset became the Element of Redemption; she is the magic equivalent of that world; she must merge with the others. To take on threats bigger than they can handle by themselves, they first stand together and activate their respective elements then they can merge together. This makes them significantly stronger, but they're not invincible. They have limits, and the more they use their transformation, the stronger it'll get. It's also crucial that they keep in sync with each other; this means their bonds of friendship have to be strong enough to maintain the longevity of their transformation.

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Redemption is a process, not a trait, and the elements are rooted in traits. So you might find this to be a hard sell as it were.

What's more, EQG stopped using the "elements" rather quickly.

I can only suggest looking at another route for introducing the concept. Harmonious is an interesting idea, but it needs a sound execution.

Right now, it doesn't come across as being on entirely stable ground as it were.

The only advice I can give is to keep working on the idea Harmonious and to not get ahead of yourself.

Trust me when I say that you don't want to be shortsighted with it. I can speak from experience that it hurts what you're trying for.