Together

Greninja and Ash had a special connection. That much was obvious. They created an incredibly powerful Pokèmon that could take down the strongest Mega-Evolved Pokèmon. What was less obvious were the side effects that came with that connection.

The first started right after the rescue of Chief Hanzo. On the way back to Ninja village, Ash noticed that his eyesight seemed sharper. The dark path they'd followed earlier was much easier to see, each dip and root revealing itself to him. He could see Pokèmon sleeping in the trees and the bushes, snoring softly in the deep of the night. He could see each strand of fur on Braixen's tail, bristling in the wind.

His hearing was much better as well. That night, as he tried to fall asleep, he could hear every ninja in the village. Some were sleeping, some were talking in hushed tones as they took down decorations from the celebration, and some were walking around as guards for a night watch. Bonnie's snoring was deafening in his ears, even though he knew she wasn't being loud. It was quite soft actually, but it felt like an earthquake rattling his brain. Was this what Greninja went through all the time? It couldn't have been. Even Greninja seemed agitated that night, glaring lightly at Chespin's Pokèball.

The senses never faded, but Ash grew to live with them. It helped them when Team Rocket tried to attack, and it aided him greatly in gym battles. He could see every move his opponent made, no matter what it was.

That leads us to the second effect. Ash first noticed it during a battle with Sam, a trainer he'd met on Route 72. Halfway through the battle, when he and Greninja were one, he noticed a soft spot in Shiftry's side. In fact, he could see a soft spot in every Pokèmon. It was usually small, but it glowed like a neon sign in a dark void to his eyes. Sometimes he could see it without Greninja, although it usually had to be a pretty intense battle. It was one thing he was happy to keep exclusive to Greninja, oddly enough. The bright glow could sometimes be distracting during battles.

The third, and the weirdest in Ash's opinion, started after their most powerful bond before the Kalos League. For a split second during the battle, when Ash got the feeling that he and Greninja were as combined as they could get, he swore he heard his Pikachu taunting Charizard. Not just a little Pika, or a gesture that Ash knows he didn't teach Pikachu, but honest to Arceus words coming out of the Pokèmon's mouth. It almost throws him off his rhythm, but Greninja assures him that's normal for Pikachu and they move on. For a few minutes, Ash chalks it up to the heat of the battle. Words were being thrown around by everyone, and it would make sense for something to get mixed up in his brain long enough for Pikachu to talk.

He's reminded of it later, once they'd settled in a nice clearing for the night, when Chespin and Pancham start to argue. They'd been arguing for days, driving their trainers crazy because they couldn't figure it out. There was no reason for these Pokèmon to be fighting, yet they were squabbling day and night. Ash was tired at this point and had a headache that was just not going away, but it still startled him when Pancham started accusing Chespin of stealing the last Pokèpuff three days earlier. Ash is unable to do anything but stare, his brain trying to catch up to the fact that he can perfectly understand two tiny Pokèmon that he's never been able to understand before.

"You were supposed to share, you jerk! I only got one Pokèpuff, and you got three!" Pancham angrily growls, pushing Chespin hard enough that it falls.

"It's not my fault you were too slow! Like I said, I thought you'd gotten two already! It wasn't my fault you stayed silent when I grabbed it." Chespin leapt up. The two stared at each other for a few seconds before they started trying to wrestle the other to the ground.

"Oh, no. Not again." Serena sighed, reaching down and grabbing Pancham.

"Come on, you two. Will you quit fighting already? I'm sorry about Chespin, Ash. I know your head must be hurting really bad." Clermont looks apologetic for a short bit, but he's snapped out of it by the shocked look on Ash's face.