Welcome Poke-fans. Since this is one (if not THE most) popular fanfic type out there, I decided to try my own. This is my first Pokemon fic ever, so it's probably not gonna be that great... I don't have high hopes for it. Let me start by explaining whats going on:

1) This story will explore a "What If" scenario, if Greninja went with Ash (yes, I know this has been done once before, but hear me out!). Personally, I was a little pissed the anime let him go, the writers seem to do that a LOT when Ash gets a powerhouse other than Charizard (ironically, Greninja is a contrast to him if you really think about it), but I understand cause that's just the way Ash is. But still, the negative-energy-in-the-plants thing was just a BS way to take him off the show, my opinion.

2) Similar to above, I was upset they let Serena go. I honestly thought she was gonna be a Misty... ya know, follow Ash around for another season or 2 THEN try to be a Pokemon Performer. Then again, I'm not totally surprised. That aside, she is gonna go with Ash to Alola as well, so you can think of this as an Ash x Serena story with some Greninja x Braixen. Pokemon reflect their trainers feelings, right? :P

3) Despite my feelings on it, I'm gonna use the negative-energy-that-Greninja-can-see-in-the-plants thing for a plot device. I don't wanna reveal what's its for, but it has something to do with the... "events" that happen in Alola. That's assuming this story will catch your attention but again, not hopeful.

4) Ash, as well as his friends and classmates, will be at the minimum age of 14 in this story. Now that doesn't mean I have a problem with his "immortally" in the anime, but since the show had 3 Xmas specials and Pikachu's Anniversary, I'm fast-forwarding time here. Trying to be realistic... well, as realistic as you can be in fantasy ;)

5) This takes place after Ash got home from Kalos. A month later, Mimey won a free trip to Alola via lottery. That's what its gonna seem like at first, but there's a reason Ash's family was the one to win it. Likewise, Serena will be in Alola (since she has a cameo appearance in the Sun&Moon games... you know what I'm talking about) because during the time-gap, she got upset and quit. Plaermo understands her feelings, by remarking she felt a connection between Serena and "that boy". However, her showcase was scheduled in Alola and the flight was already paid for, so basically Serena gets a free trip!

6) While at the airport, Ash in his excitement runs into Serena not realizing its her at first. Until he reaches for a certain hat that fell of her person. They of course had a staring contest and then a hugging moment, with Serena sinking into it and not caring if anyone saw. They had to explain to Delia and Oak (who Delia ran into cause he was visiting his cousin in Alola) who she was, with Delia embarrassing Ash by saying "oh, so your the pretty girl he keeps talking about."

7) I won't be following anime exactly. I will include some of it, but the games are mostly what this storylines gonna be based around, and some original stuff. Yes, that means both regular and Ultra Sun&Moon.

Other than that, I'm aiming to make a plausible plot as to what could have happened anime-wise if Greninja was allowed to stay in the roster. Also unlike my other stories (if your a reader of them), this one won't be given much attention unless I get a decent amount of reviews/follows/favs. Any reviews: good, bad, suggestions, criticism, etc. I really hope you all enjoy but again, I'm not exactly expecting much. If this story is bad, say something! I'm a big boy, I can take it!

Oh, and before I forget, I have a goal in mind. I haven't noticed any Pokemon fics with Ultra Beasts, not even of Ash catching one, which stunned me. I've seen Ash get shiny, Legendary, Mythical... but not one Ultra Beast. I plan to have him get one, but that'll be up to the lot of you if this is giving good ratings/traffic.

Narrator:

When we last saw our heroes, they had tearful good-byes to pursue their own dreams. Among them, was the highly unexpected departure of Greninja, whom stayed in Kalos to help the 2 Zygrade; Squishy and Z2. Some of the corrupted energy from Lysandre's plan infected the wildlife, causing roots to spiral out of control. Neither Zygarde could easily track down the inflected land, but Greninja's mysterious powers were able to locate them instantaneously. It wasn't easy, but Ash did the honorable thing and released Greninja under the care of Zygrade, while giving each other a tender hug farewell.

Ash has had bonds with his Pokemon before, but none that manifested itself quite like Greninja's did. So much so that even after returning home, he was depressed for a week afterward. He didn't sleep or eat... heck, he didn't even battle Gary who challenged him with a new Pokemon he obtained recently! Gary, Professor Oak, Tracy, Mimey and Delia asked if anything was wrong and while Ash claimed he was fine... Pikachu knew the truth. Even Charizard was sensing something amiss with its trainer, but kept to himself about it. Ash did get over it after hearing they won a free trip to overseas... to Alola! He's heard of the location all his life and always wanted to venture there... especially cause of all the Pokemon he could meet and new friends he could make.

And so... enjoying a relaxing vacation from all the world-saving and badge-collecting; Ash, his mother, Professor Oak and even Serena whom Ash happened to bump into at the airport; could do with some sunshine and spending quality time together. But what of Greninja? How is he adapting to his new lifestyle in the past month? Well, why don't we just drop in, and find out!

-SONG START- (AN: You know, THAT song! I do not own it, just modified it to fit the mood and setting)

(Koko Tapu flies around in the sky. An unidentified Pokemon flies beneath it, appearing from a... hole?)

I still wanna be, the very best! That no one ever was.

(Ash looks up to the sun, it shows he's looking up at some kind of league.)

To catch'em all, is my real test! And to train them is my cause!

(Ash catches a Pokemon and gets excited. He goes to let it out, but lets Charizard out accidentally, who gives him a "kiss".)

I've traveled, across the land! Been searching far and wide!

(Ash runs with Pikachu and his classmates. Some Pokemon fly around them, which gets Lillie nervous and hugs Ash for protection. Everyone sweatdrops... except a certain french girl, who gets red and grows a forehead vein.)

Teaching Pokemon, to understand, the power that's inside!

(Ash and Greninja fight a Tyranitar, with water exploding from Greninja's body, causing his well-known transformation.)

POKEMON! GOTTA CATCH'EM ALL!

Still you and me... I know its our destiny!

(Ash and Pikachu hug one another. Then, the scene changes to a battle, with said rodent executing Iron Tail on his opponent.)

POKEMON!

Oooooh, your my best friend, in a world we must defend!

(Ash and friends cheer Pikachu on, but then the sky opens up... what are those things flying out?!)

POKEMON! GOTTA CATCH'EM ALL!

Our hearts are true! Our courage will see us through!

(A creature of some kind interrupts the battle and attacks Pikachu, who dodges its assaults then responds with his signature move: Thunderbolt.)

You'll teach me, and I'll teach you! PO-KE-MON!

(Lillie tries to confront her fear by petting Greninja, but reels back. Greninja understands and is very patient with her, until she finally touches him and realizes this isn't so bad. Her phobia finally conquered, she gives Greninja a hug, which he returns. Everyone else crowds around them, making a group hug.)

GOTTA CATCH'EM ALL! (GOTTA CATCH'EM ALL!)

(Ash throws a Poke-Ball, releasing an unidentified Pokemon... the same one that was flying under Koko Tapu earlier. It gives a battle cry, ready to fight!)

POKEMON!

-SONG END-

Ash: (instrumental music plays in the background) A Friend from Afar! - Part 1

In the Kalos region, we see a Greninja leap out from some trees into an open field. He looks around, as if sensing something... that's when a giant root erupted from behind him! Using Cut, he sliced the destructive plant. More roots flew at him, so he spun around, slicing them as well. An even bigger root headed for him from in front, that's when Squishy (10% Forme) used Extreme Speed to ram into it. Another root erupted, this one was bigger than the rest. Z2 (50% Forme) arrived on the scene and disintegrated it using Dragon Pulse.

The 3 warriors then were back-to-back, waiting if anymore roots would pop up. Sensing no more negativity in this area, Greninja gave an "all-clear", so the trio could relax. Finding a safe enough spot in the shade, Squishy released its cells and changed back to normal, apparently he gets tired much easier than Z2 does. The red Zygarde core hopped on a rock that had a beam of sunlight, and laid down. Greninja just leaned against a tree, had his arms folded, looking up at the sky.

This wasn't the first time Greninja did this, ever since his trainer left him under their care, both Zygarde noticed the frog-like ninjas behavior. "Poor Greninja, he truly misses Ash" said Squishy. "He was called the Chosen One by Lugia years ago, it only makes sense" said Z2. Squishy was deep in thought upon hearing this, as a Legendary himself, he knew of Ash's status. "I think we should let him go" said Squishy. "Your not serious, are you? Greninja is our best chance at..." Z2 tried to argue, but Squishy met the blue-cored Zygardes gaze.

"He's done more than enough, we've traversed the entire Kalos region the past few weeks, and there's barely any negative energy left! Greninja has kept his end of the bargin and besides, its our duty as a Legendary, not his" said Squishy adamantly. Greninja overheard the conversation and while his heart skipped a beat, he still couldn't find it in himself to just abandon his home. He told this to both Dragon/Ground-Types, but they (in Z2' case, reluctantly) assured they could handle it, it was time for Greninja... to return home.