Catch the latest, cherish the timeless
Day 1: Less patient Ash, I believe every time Ash and Greninja connect, Ash's eyes have a red outlining and greninja has a brown one
I was supposed to do them all but a lot of trial and error got on my nerves so I'm doing one every other day
Whereas Greninja takes on physical features to Ash, Ash takes on more of Greninja's personality when they're fused. He becomes quieter, more focused, less cheery. He's also a lot less patient. If you try to distract him, he'd probably tell you to shut up (Ash would apologize afterwards though.) His voice also gets a little bit deeper and softer.
This is also because Ash easily get overloaded when he's hearing or seeing things that doesn't line up with what Greninja's sensing. It won't be super obvious during the battle, but after Greninja reverts to its original form, Ash will often just stand there, his brain trying to sort out everything that happened.
Ash can feel all the pain Greninja's feeling, even in base form (it's significantly less though, more of an annoyance if anything.) Greninja can only feel Ash's pain if the injury is severe.
It's the exact opposite for their emotions. Because Ash is generally a very emotional person, Greninja can almost always tell how Ash is feeling. Meanwhile, Greninja is more calm and reserved, so Ash only feels really severe emotions.
Although they don't share their senses all the time, Greninja can feel when someone's hugging Ash. It may or may not be intentional on Ash's part.
Greninja will often ask Ash about human behaviors. Greninja is always reminded that Ash is the last person you should ask about human behaviors.
The two of them always share a mental link, no matter how far they are from each other. They technically can do the bond evolution from anywhere, but doing it when they're not in the same place often ends up with Ash being overstimulated or frozen in place.
Ash doesn't need to give orders when they're bonded. He goes deathly quiet and just copies his moves.
Ash once tried to use a Z-move while bonded to Greninja. He fainted. (He probably could pull it off, it'd just take a lot of practice, and Greninja thinks he's plenty powerful already.)
If Alain is in Kalos, then Greninja often chases him down and battles with Charizard. These battles are usually causal, but if Ash tunes in, then things start getting heated.
their friendship is so real to me