Just for anyone who doesn't know, and only knows the series by Brawl, Mewtwo, Young Link, Pichu, Roy, and Dr. Mario were all characters who appeared only during Melee. I wish they'd kept them.
Also, I use only characters that were present in Melee.
This is my theory of what happened to the characters that were left behind in Melee. Yes, it's pretty tragic, but what else?
He hated everyone.
He hated everything.
He hated his creator.
And, most of all, the person who's DNA that he was built out of, Mew.
The fury. It consumed him. He wanted to fight, destroy. Which is why he considered the only good thing that had happened to him was when he'd received a visit from the cave he was residing in.
Dr. Mario threw a pill at Mewtwo, dodging the carefully timed energy ball. The Legendary Pokemon flinched at the impact. He raised a hand, and the Dr slammed against the Floor of the Cave. Zelda came running behind, and fire came from her hands, slamming into the back of Mewtwo's head.
He turned around to destroy her, but she teleported away, fast. Young Link appeared from the Mouth of the Cave, and he slashed at Mewtwo's tail. The Pokemon winced in pain. Who were these fighters?
Eventually, a force that consisted of Zelda, Dr. Mario, Young Link, Mr. Game and Watch, and Pikachu was able to detain him, and try to explain.
"We're here, because Master Hand sent us to recruit you for the games." Zelda told him. Mewtwo's eye widened in surprise.
"You…work for that freak?"
Young Link snorted. "Look at yourself. You're not exactly the average Hyrulian either, bub."
Mewtwo telepathically slammed his captors against the sides of the caves, and held his cold, white hand to Young Link's throat.
"You dare insult me with your hideous…slang?"
Ganondorf dropped down into the cave. "Looks like this one's got quite a bit more power than the last we recruited." He looked threateningly at Mewtwo.
"Let. The. Boy. Go."
Zelda smirked. "Since when do you care for Link or his younger counterpart?"
A glare was sent in her direction. "I care for whether Master Hand destroys me for losing one of us."
Mewtwo hesitated. Even he could tire out eventually, and he was far outnumbered. He released his grip from Young Link, who proceeded to hide behind the figure of Zelda, who comforted him.
Ganondorf sighed. "Let's get this…ordeal, over with."
Mewtwo looked at the tall, ugly, but menacing man. They could surely not have anything of interest to him, could they?
"Master Hand, that so called 'freak' is requesting you to… join the Melee Tournament. Are you in? If you're not, we have to destroy you. Policy."
"Pika Pika!" Pikachu answered.
Long story short, he joined, and fought brilliantly. Sure, he was overpowered in some cases, but at least he was far from the weakest fighter there.
When the tournament ended (Oh, probably around 2008, lol) he promptly expected that he would be drafted into the next one.
He didn't expect that he'd be replaced with some blue cat like thing. (Yes, I'm a big Lucario fan, just think from Mewtwo's point of view)
He also didn't expect he'd be just…brushed aside, and destroyed.
The Wire Frames, Master Hand's personal army, were arriving in the small cave by the dozens. Anymore and there would be nowhere to stand.
"YOU THINK TO DESTROY ME? YOU THINK, THAT I'LL LET THAT FREAK-ISH HAND GET THE BETTER OF ME?"
Energy Ball after Energy ball, punches after punches. He slammed 4 against the massive horde, sending a few dozen out of the cave. Only a few remained.
They were quickly defeated after that.
Master Hand sighed from the video feed he was getting. He pressed a button to summon Giga Bowser, his best weapon.
Let's just say it wasn't pretty.
Pichu wandered happily through the field, with his friends Jigglypuff and Pikachu. It was just a happy day, and he had some real fun fighting in the tournament.
Some Metal Cages fell from the sky, with no floor, falling right onto Jigglypuff and Pikachu. The floor materialized below their feet. Pichu's eyes widened in fear.
"PIKA! PIKA!" Thunderbolts struck Pikachu's cage, to no avail. Jigglypuff double-slapped the bars of the cage. It was no use.
Some more Wire Frames appeared, to destroy Pichu.
Pichu let loose a bolt of electricity to buy him some time. He could escape into a tunnel, and wander around the underground.
Dr. Mario fought hard.
So Did Young Link.
As Did Roy. But it was no use.
Master Hand nearly always got his way.