The Return of Greninja

Panic At The Pokemon Center

Ash was on his way to the beach to check for more signs of the unusual Pokemon that had been spotted just a few days ago. He still couldn't help but wonder if the mysterious Pokemon was his Greninja, even though he knew how far-fetched that would be. How in the world was Greninja supposed to leave Kalos? Pokemon couldn't travel without trainers! And even if he could, what was the chance that he would just happen to end up in the same region Ash happened to be in? Unova was far closer to Kalos than Alola; if anything he would have ended up there.
"Hey, there! You in the hat!" a voice shouted from behind.
Ash turned around to see a boy about his age running up to him. "Let's have a battle!" the boy said. "My name's Julian. I come from the Sinnoh region." 
"Pika pika," Pikachu said, waving.
"I'm Ash, nice to meet you," Ash greeted him. "All right, you're on. How does one on one sound?"
"Fine by me. Go Torterra!" Julian shouted, tossing out his first Pokeball.
"Torterra, huh," Ash remarked, recognizing the Sinnoh grass-type starter Pokemon. "In that case, then...Hawlucha, I choose you!"
There was a flash of light as Hawlucha emerged from his Pokeball, striking his signature pose and giving a battle cry. Julian looked at the forest champion curiously.
"Hey, that's a new one," he said, pulling out his Pokedex and scanning Hawlucha with it.
"Hawlucha, the wrestling Pokemon. With its wings, it controls its position in the air. It likes to attack from above, a maneuver that is difficult to defend against," the Pokedex recited.
"A fighting and flying-type," Julian muttered, seemingly to himself. "All right then, Torterra, use Energy Ball!"
"Dodge it, Hawlucha!" Ash ordered.
Hawlucha easily sidestepped the Energy Ball, sending a small smirk over to Torterra as he did so. "Now, Hawlucha, use X-Scissor!" Ash shouted.
"Lucha!" Hawlucha replied as he crossed his arms in front of him. They glowed silver as he rushed towards Torterra, slamming into him and sending the bulky grass-type onto his knees.
"Torterra, are you all right?" Julian asked worriedly.
"Tor," Torterra replied, wincing. Nevertheless, he gave his trainer a reassuring smile.
"What move was that?" Julian exclaimed. "I've never seen it before!"
"X-Scissor is a bug-type move," Ash told him. "That's why it did so much damage."
Julian nodded and gritted his teeth. "You're a strong trainer, Ash. But I'm not letting you win this," he declared. "Torterra, use Hyper Beam!"
"Flying Press!" Ash countered.
Hawlucha soared into the air, easily evading Torterra's Hyper Beam. While Torterra was still recharging his energy after using the powerful attack, Hawlucha spun around in the air before speeding down towards the grass-type, crashing into him and knocking him out. He landed back next to Ash and flexed his muscles triumphantly.
"Torterra, no!" Julian exclaimed, running over to check on his fallen Pokemon. "You were great. Return."
"Great job, Hawlucha," Ash praised the hawklike Pokemon. "Return too."
"Hawlucha," Hawlucha said happily as he was sucked back into his Pokeball. Ash put his Pokeball away and went to go talk to Julian.
"Your Hawlucha is pretty awesome, Ash," the Sinnoh native stated. "That was a great battle."
"It sure was," Ash agreed. He held out his hand. "What do you say we battle again another time?"
Julian smiled and shook his hand. "It's a deal," he replied.
Ash smiled back and was about to say something when Lillie suddenly ran up to them, gasping for breath. There was a look of terror in her eyes and Ash immediately feared the worst. Was Team Skull up to their old tricks again?
"Lillie, what's wrong?" he asked in concern. "What happened."
"Ash, you have to come with me. There's a Pokemon going completely crazy in the Pokemon Center," Lillie gasped out. "Hurry, follow me!"
Ash exchanged a glance with Julian and nodded. The two boys ran after Lillie to the Pokemon Center that was located about a block away. As they ran, he decided to ask Lillie what was happening.
"So what happened, Lillie?" he asked.
"No one knows. Apparently it's that mysterious Pokemon someone saw on the beach a few days ago. He just started going nuts," Lillie answered. "Professor Kukui sent me to find you."
"What could have happened to make him react like that?" Julian asked.
"I don't know, but he's causing a lot of damage. We need someone to help calm him down," Lillie replied as they came to a halt outside the Pokemon Center. "Come on, in here!"
Ash followed the blond-haired girl into the Pokemon Center. At once, he was met with a familiar yet strange sensation as though something in his chest was threatening to break loose. But it was connected to something...a Pokemon. A Pokemon that was very familiar to him, although it had been more than two years since he'd seen him.
"Greninja," Ash whispered.
Without even bothering to wait for Lillie and Julian, Ash ran towards the back of the Pokemon Center where all the patients were kept. As soon as he burst into the door, he saw about fifteen other trainers with Pokeballs in their hand, red beams of light shooting out of them every few seconds. Some of their Pokemon were out as well and were firing attacks at random. There was a broken lamp lying on the floor and a hole had been burned into the ceiling. In the midst of it all was a very familiar bipedal frog Pokemon, dodging the oncoming attacks and launching some of his own.
"Go away!" Greninja spat at his attackers. He hurled a Water Shuriken at a Flareon and knocked him out cold.
"We have to get him under control! Gardevoir, use Psychic!" a man shouted.
"Stop!" Ash shouted. "Pikachu, Thunderbolt towards the ceiling!"
"Pika-CHUUUU!" Pikachu shouted, firing off a powerful bolt of electricity. It didn't hit anyone, but it was enough for Ash to get everyone to stop attacking Greninja.
"Hey! What are you doing?" the man with the Gardevoir snapped. Ash ignored him and ran up to Greninja, who was still crouched defensively, his arms ready with Cut in case anyone decided to make a move.
"Greninja, it's okay. You're fine. Everything's fine," he said, placing a hand on the frog-ninja Pokemon's shoulder. "Greninja, look at me. It's all right now."
Slowly, Greninja met his gaze. As their eyes locked, the anger and fear in the frog Pokemon's eyes faded and were replaced by recognition, relief, and happiness. He lunged forward and wrapped his arms around Ash, nuzzling him happily.
"That's right," Ash murmured, "that's right."

So Ash and Greninja are finally back together! As for why Greninja lost control like that, imagine how you would feel waking up surrounded by strange people in an unfamiliar place. I know I wouldn't be too happy.