This idea was suggested by @Mastermind64. Thanks for the suggestion!
Third Person POV
Ash is alseep on the couch in the loft that serves as his room. The moon shines through the skylight on the roof onto the sleeping teen. Suddenly, a certain Moonlight Pokémon makes a void in the moonlight.
The eeveelution starts tapping the window with one of her paws. The tapping causes a certain electric type to shift his ears before opening his eyes. Pikachu uses his paws to wipe at his eyes, so he can see.
All the while the tapping never ceased. Letting out an annoyed 'pika', Pikachu looks towards the skylight. Immediately recognizing his friend, he becomes less annoyed.
Spotting the letter in Umbreon's mouth, Pikachu then starts to wake up his best friend. After multiple failed attempts that either ended in annoyed grumbles from his trainer or no reaction, Pikachu does the one thing that he can do to wake the teen up. He uses thunderbolt, but of course not full power because he doesn't want to wake up Rotom Dex, Kukui, or the other Pokémon.
Finally, Ash wakes up from the electric shock that he's become used to and immune to the pain that it should cause him. He rubs his hand over his eyes.
"What is it bud?" The Kanto trainer quietly asks his partner.
"Pika." Pikachu replies just as quietly and points to the skylight.
"Umbreon?" Ash questions with a frown once he spots his boyfriend's Pokémon.
Said Pokémon's eyes glow, the telltale sign of her using psychic, and the skylight opens. Umbreon releases the letter from her mouth letting it fall to the human and Pokémon.
"Breon." She says then closes the skylight and leaves.
Ash and Pikachu look at the letter that has 'Ashy boy' written on the front before both shrug. Ash opens the letter as Pikachu climbs on his shoulder, so they can read it.
Ashy,
I missed spending time with you, so today was wonderful. Without a doubt, I know you missed seeing me too. Being alone with you and catching up reminded me of the past when we ran into each other in Sinnoh.
After I go back to Sayda Island, I don't know when we'll next talk let alone see each other. Don't let girls kiss you because you know something will happen once you tell me.
Arceus I'm so fucking bad at this. I don't really know why I'm writing a love letter to you and I haven't even written down the three words this thing is supposed to have. I guess what I'm trying to tell, or really write, you is that I love you which is weird because we argue, get into each other's faces, and we flirt by insulting each other.
But I guess I'm writing this because we could probably say 'I love you' more instead of in the rare moments when we are together.
Smell ya later,
Your boyfriend, Gary motherfucking Oak
When Ash is done reading the letter, he has the biggest smile on his face. He grabs Pikachu and hugs him tight. Sure when it got to the part about the girls, he shuddered knowing that his possessive boyfriend would do something bad landing him in jail, but he still loves the researcher.
After letting go of his partner, Ash hides the letter under the couch he sleeps on. After a few minutes of Pikachu already being asleep, Ash falls asleep still grinning.
Time Skip
Ash's POV
I wake up to Rotom shaking me. "Ash you need to get ready for school!" He says.
After reading the love letter Gary sent me, I'm not even upset about being woken up because I hate being woken up. I get into my usual clothes and head down my loft with Pikachu hanging on my shoulder.
"Morning professor!" I happily greet him.
"Morning Ash." Kukui says slightly surprised about my happy mood.
We eat breakfast, and I can feel Kukui and Rotom's eyes on me. "Take a picture it lasts longer." I tell the two blankly then I look up at them.
Kukui looks away from me with an embarrassed blush on his cheeks while Rotom Dex still stares at me.
"Sorry Ash." I hear Kukui apologize.
I sigh feeling a bit guilty. "I just don't like being stared at." I tell him, and it's true. It took me a three months just to get used to Brock and Misty staring at me every once and awhile. I never had to get used to Gary, Professor Oak, or my mom staring at me because my mom is well my mom, I grew up with Gary and we had staring contests when we got super bored, and Professor Oak watched us quite a bit when we were kids and since sometimes we were in the lab he had to make sure we didn't mess with anything.
After I finish breakfast, Kukui tells me I can go ahead to the school without him. I make sure my Pokémon are done eating before calling them into their pokéballs.
"See you at the school, Professor." I tell him.
"See you later Ash." He says then Pikachu, Rotom, and I leave.
We take it slow since we don't have to rush there. Sadly though, Rotom asks me questions about Gary's research and I try to keep up but I couldn't answer some questions since I don't know the answer.
"Rotom." I say cutting off his question. "I don't know some of the answers, and you're not giving me enough time to answer the questions I do know the answer to. Just ask Gary your questions."
"How can I ask Gary questions when you run off without me to Gary?!" He asks me annoyed.
"You got me there." I say as we get the the school grounds. We get up to the classroom to see everyone but Mallow there. "Alola." I tell them.
"Alola." They greet.
"So Ash, what did you do yesterday on our day off?" Lillie asks.
"I sent more time with Gary." I shrug. "What did you guys do?"
"I do what I always do." Kiawe says. "Did my deliveries and worked on the farm."
"Fishing of course." Lana answers.
"I went with mother to help at Aether Paradise." Lillie says with a smile.
"That's cool Lillie." Lana says.
"I worked on my programs." Sophocles finishes off answering my question.
"Is Mallow okay?" I ask.
"We don't know." Lana answers a bit worried.
Professor Kukui walks in greeting us, and we greet him in return. That's when he notices we are short one person.
"Where's Mal-" Kukui starts to ask before we here footsteps running towards us.
"Sorry I'm late!" Mallow exclaims running in the classroom. She stops to catch her breath. "The kitchen was a mess and we helped my dad clean it." Mallow says gesturing to herself and Steenee.
"It's fine Mallow." Kukui tells her with a reassuring smile. "Just please try not to be late again."
"I'll try." She says as she goes to her seat with her Pokémon following her.
Time Skip
After school is over, we decide to play a game hide-and-seek since we're bored and we want to be kids right now. Kukui already went back to his house, so he doesn't care about us playing a child's game as long as no one gets hurt. We also decide that Rotom Dex stays with Mallow because it would give itself and whoever it's hiding with away.
"Are you guys sure you want to play this?" I ask them. "None of you will be able to find us." I say as I rub Pikachu's head.
Since Gary and I were the only kids in Pallet Town, it sometimes got boring when it was just us playing. So, we'd hid from my mom and Gary's grandfather making them go crazy looking for us. They wouldn't find us until they asked the Pokémon at the Oak lab to help find us because it was always the Pokémon that found the two of us.
I shake out of my memories when I hear Mallow's determined voice. "Of course I'll find you. Bet I'll find you and Pikachu first too." Since Mallow was late we all agreed that she would be the seeker.
"You won't win that bet." I say simply and Pikachu nods his agreement.
"Close your eyes already and start counting!" Sophocles says.
"Fine!" Mallow says. "Remember no hiding in town or past going past Oranguru's Place."
"We know!" The rest of us say.
"Okay then!" Mallow closes her eyes and turns around. "1...2...3..." she continues counting and we all run away from the green haired girl to hide.
I don't pay attention to where the others are going to hide. I enter the forest looking for the best place to hide. After a walking around, I find the perfect tree to hide in. The tree is bigger than the other trees around and the trunk is thicker, and there are plenty of leaves to hide us.
Pikachu climbs the tree before I follow. We climb as high as we could without the branches breaking from both of our weight. I get comfortable knowing it's going to be a while before we're found. Pikachu lays on my lap as I pet and scratch his head, back, and tail.
Time Skip
"OKAY ASH COME OUT NOW!" I hear Mallow yell. It's been two hours since we stared and Mallow found everyone but Pikachu and I.
I silently laugh while calming Pikachu from being woken up from his nap. I can relate Buddy. I see Lana and Popplio walk by looking up in the trees. I smirk knowing that the only way for her to find me is for her to climb up this tree.
Time Skip
It's been four hours since we started playing and Kiawe went to get Kukui an hour and a half ago. So now my classmates, their Pokémon, my Pokédex, and my teacher are looking for Pikachu and I.
"I wonder what Gary and Umbreon are doing." I whisper to Pikachu he is munching on a berry he went to get.
"Where is he?!" I hear Lillie ask frustrated and worried.
"What are you guys talking about?" I hear Gary's voice, and that's when I decide to watch the group.
"Well, we're playing hide-and-seek and we can't Ash and Pikachu. It's been four hours!" Sophocles answers.
That's when I hearing laughing from Gary and Umbreon. "No wonder she's worried!" Gary says after he calms down.
"This isn't funny!" Kukui says with a bit of anger.
"He told you you wouldn't find him didn't he?" Gary asks, and Mallow nods. Gary laughs a bit more and now it looks like Kiawe is starting to get ticked off at him. "It's a good thing I showed up then because none of you would be able to find him!"
"I'd like to see you try to find him." Mallow says angrily.
I can see Gary smirk. "Gladly." He says and then snaps his fingers.
"Umbre!" Umbreon says then uses shadow ball, right at Pikachu and I.
Pikachu jumps on my shoulder then I jumped out of the tree. I land with of my right knee, left foot, and hands on the ground.
"Hi Gary." I say.
"HOW DID YOU KNOW HE WAS UP THERE?!" Kukui yells at Gary as I stand up and Pikachu jumps off my shoulder.
"I grew up with and only him." He answers and received weird looks. "We were the only kids in Pallet Town, hell we're probably still the youngest from Pallet. Anyways, since it was just the two of us we would get bored sometimes and hide from Gramps and Delia." He says with glazed eyes. "It took them hours to find us, and even then it was the Pokémon at the lab that actually found us."
"Good aim, Umbreon." I tell the black and yellow eeveelution.
"Umbre!" She says then puffs her chest out proudly, and Gary pats her head.
"ASH KET-" Kukui starts to yell at me before I cut him off.
"Save it, I probably heard it before." I tell him then stretch and pop various body parts, and me cutting off Kukui shocks all of them except Gary. "That's why I asked you guys if you were sure that you wanted to play this game."
"At least you were't the one finding them." Gary says with a snort.
"Oh, yeah. Game over in a mater of a minute." I say. "Considering it's been four hours I'm sure we're all hungry, we'll except Gary, so let's go eat." I start to walk out of the forest causing them to get out of their state of shock.
Time Skip
After getting out of the forest, we all split up for dinner because my friends' families are most likely worried about them. Kukui invited Gary to have dinner with us, so he can ask him questions.
Right now we have the tv on as background noise as Kukui and Rotom ask Gary questions while I just sit and watch.
"Gary how long have you been a researcher?" Kukui asks me.
"A few years." He says. "About three and a half years."
"What project are you working on now?" Rotom asks.
"None." Gary answers. "I came here after I finished my last one."
"Umbre." Umbreon says tugging at Gary's pants. Gary picks her up and sets her on his lap, and she lays down.
"Been to Pallet recently?" I ask him.
"Two months ago actually." He says. "Gramps wanted me to help him on a research paper."
After a few more questions that I can tell Gary doesn't want to answer, I look at the time. "Gary shouldn't you be going, it's pretty late." I say gaming the three's attention.
Gary gives me a grateful look. "Yes I should be." He picks up his sleeping Pokémon carefully. "Thank you for inviting me over." He says to Kukui.
"It was nothing! It was nice getting to know one of Ash's friends, especially one that's a researcher." Kukui says. "Goodbye Gary." He says.
"Bye Gary." Rotom says.
"Bye, Professor Kukui, Rotom Dex." He say to them.
I walk towards the doo and open it for him. He walks out and I step out with him then close the door. He turns around to look at me.
I give his lips a light peck. "Thanks for the letter." I say with a small smile and a blush.
He smirks. "You're welcome." Then he laughs lightly. "I still don't know why I fucking wrote that."
"Well, I'm glad you did." I say and give him another quick peck on his lips. "Goodnight Gary."
"Goodnight Ash, smell ya later." He says and starts walking off to the motel he's staying at.
Time Skip
Third Person POV
Like usual, Rotom wakes Ash up. After Ash leaves the loft, the flying Pokédex notices something white under the couch Ash sleeps on.
Curiosity getting the best of it, Rotom gets closer to it. Then he realizes it is a letter. It sees 'Ashy' on the front of the letter, and was about to go ask Ash what it was about when it dropped the letter. When the letter landed, it is open.
Cliffhanger! Have fun dying of suspense until I post the next chapter that I don't know when I'll have it done and posted. ALSO TWO CHAPTERS IN ONE DAY!!! WOO HOO! HAPPY DAY TO YOU READERS! This is my longest chapter so far too!