When they arrived on Mossdeep, Ash went straight to the Pokémon Centre – then got hold of all his Hoenn Pokémon, and got them together for a chat.
"So, guys," he said. "Or, you know… whichever. I'm wondering who to use in the gym here."
"Which gym is it?" Grovyle asked.
"Psychic," Ash replied. "And then it's Water on Sootopolis. Obviously I don't know the rules yet, but… I think it's probably going to be Marshtomp for one of the Pokémon I use, because he hasn't had as many gym battles as anyone else yet. I'm just not sure who apart from that."
"What about me?" Miraidon asked. "I do not currently know any attacks apart from Electric Terrain."
"That's… a problem," Ash agreed. "If you had a move or two apart from Electric Terrain, then it'd be great, but as it is… what about if we aim for you to get started with gym battles in Sootopolis?"
"That would be good," Miraidon decided.
"Then that's what we'll aim for," Ash said. "Apart from that… Swellow, I probably won't be using you, because you've got a good amount of experience already."
"I guess," Swellow shrugged. "Counting the stuff on that island where we did the same gym twice, I did do four of them."
"What about Unown?" Absol said. "How are they doing?"
"That's a good point…" Ash admitted. "I should use Unown either this gym or next. Thanks for pointing that out, Absol… you know, you're Dark-type so it could be a good gym for you to get involved here, but that's not needed."
"Hmm," Absol mused, tilting his head and thinking about it. "Well… I don't see anything going wrong if I take part, but then again, all that really means these days is that things will eventually work out."
He shrugged. "It's very relaxing, but it does mean I can't tell if it would be better for me to battle or not."
"Actually, now I think about it…" Ash frowned. "I guess it might be up to you if you'd rather do this gym or the Water one, Grovyle? Because you did put in a lot of work learning Night Slash, and it's still a good move to have, but you did do the fighting gym twice like Swellow did?"
"Working out schedules for training up Pokémon seems very complicated," Latias said.
"Yeah, it is," Ash agreed. "I… guess I've got an idea of what to do next, though."
He frowned. "Huh. Sometimes when we meet people who are doing a gym challenge, they only have a few Pokémon and I wonder why, but maybe it makes this kind of decision easier?"
Ash knocked on the door of the Mossdeep Gym, and two similar-looking young trainers opened the door.
"Welcome!" they said, then actually saw Ash, and stopped speaking.
"...uh," the boy said, eventually. "You're Ash Ketchum, right?"
"That's me!" Ash replied. "Who are you?"
"Tate," the boy said.
"And I'm Liza," the girl added. "And we're Liza and Tate!"
"Sis," Tate complained. "We agreed Tate and Liza sounded better!"
"You agreed it," Liza grumbled.
"It's what the Gym registration says," Tate defended himself. "So-"
"You're the Gym Leaders?" Max asked. "Okay, this is getting ridiculous."
"...rude!" Liza huffed.
"No, I don't mean that, I mean that I kept being told I couldn't get a Pokémon until I was ten," Max explained.
Tate wordlessly pointed out Swablu.
"I mean-" Max began, then groaned. "Oh, never mind…"
"Two Gym Leaders…" Ash said, thinking out loud. "So, which one of you do I battle?"
Tate and Liza exchanged confused glances, then both looked at him. "Huh?"
"Well, at Cerulean Gym there were four gym leaders, so I know how this works!" Ash said.
"Actually, you battle both of us," Liza told him.
"At once!" Tate agreed. "It's a Double Battle!"
"Double Battles… I've heard of those," May said. "Two Pokémon on each side, right? Or… is it two Pokémon at a time on each side?"
"Double Battles are two Pokémon at a time on each side, but in our gym it's two Pokémon only," Liza said. "It's a test of co-operation, and of course we're good at it because we're twins!"
"Are you sure that's how that works?" Latias asked. "I could ask my twin how well we get on, but he'd probably give a different answer to me."
"Hey, what Pokémon was that?" Tate said. "Liza, did you hear that?"
Latias cancelled her invisibility, and both Gym Leaders gasped.
"You're such a cool Pokémon!" Tate gushed. "Latias and Latios are one of those great twin combinations of Pokémon, and they're Psychic types! Like Gardevoir and Gallade, that's a pair we're thinking of getting some time."
"Or Meowstic and Meowstic," Liza nodded. "Anyway, pick your Pokémon, challenger! And we'll show you what teamwork really means!"
Ash had already decided to use Marshtomp, but for his other choice he thought for a long moment before selecting Koraidon.
"It's a pity Miraidon isn't ready to battle yet," she sighed. "Or otherwise we could do a sibling-sister team up like they're doing."
"Yeah," Ash agreed. "Maybe we can do some double battle practice in future so they can get a go at it. And, remember, Koraidon, sunlight only when you need it, and not when Marshtomp wants it to be avoided."
"Got it!" Koraidon said, nodding.
"Marshtomp, you might need to keep her safe from attacks that work especially well on her," Ash added. "Just getting a Rock Throw or Mud Shot in the way could be important."
Marshtomp did something that was sort of like a salute. "Got it."
Then Tate and Liza sent out a Solrock and Lunatone.
"Oh, yeah, that is another paired set of psychic Pokémon, isn't it," Max realized. "Huh! Maybe I should have seen that coming."
"I'm a bit disappointed that I didn't see it coming," Brock lamented. "They are Rock types."
"Let's do this, Koraidon, Marshtomp!" Ash said. "...wait, who's doing referee stuff?"
"I'll do it," Brock volunteered. "You don't always need one in gym battles, but in a two on two it's probably a good thing to do."
The moment that Brock called for them to begin, Ash pointed over to the side of the arena. "Koraidon, over there – Marshtomp, Muddy Water!"
Koraidon bolted over to the gym corner, and Marshtomp opened fire.
Or, rather, opened water.
The jet of muddy water hit Solrock hard, knocking it back into the wall, but both Tate and Liza were already pointing.
"Light Screen!" Liza called.
"Psychic!" Tate added.
Lunatone's point glowed, forming a psychic shield to block off a lot of the impact of Marshtomp's Water attack, and Solrock's eight points all glowed and lit up in unison.
A blast of psychic force smacked into Marshtomp, sending the Water-type rolling backwards, and Ash glanced between him and Koraidon.
"Rock Smash!" Ash told her. "Marshtomp, Muddy Water again, both on Lunatone!"
Koraidon broke into a sprint, then jumped, and her tendrils unfurled into wings.
"Whoa!" Tate gasped.
"Focus, Tate!" Liza warned.
Her wings let Koraidon get up high, as high as Lunatone was and where Lunatone had just been knocked back towards the gym wall, then she released her hold on her Ability and glowed suddenly as sunlight streamed in through the gym windows. She smacked into Lunatone with a powerful Rock Smash, one which went straight though the Light Screen, and the impact slammed Lunatone down into the ground.
"Whoa!" Max gasped.
"Solarbeam!" Tate called, pointing at Marshtomp.
"Cut the sun, Koraidon!" Ash ordered, and Koraidon turned off her Ability just before Solrock could use it to fire out a quick Solarbeam.
The Rock-type was still charging, though, so Ash turned his attention to Marshtomp. "Dig! Koraidon, try to keep Lunatone on the ground so Marshtomp can target them!"
"Lunatone – Moonblast!" Liza instructed.
The blast of silver light that erupted from Lunatone did a lot more than anyone was expecting. Ash had sort of accepted that Koraidon was probably Dragon type, given that Lance had thought that and he was an expert, but whether or not she had a second type was a mystery – and Moonblast launched her all the way back to the roof, slamming her into it hard enough to leave cracks.
"Ow!" she yelped, and Ash took half a step forwards before shaking his head.
They were in a battle-
-and Solrock's Solarbeam had fired, hitting the ground because their target was underground.
"Marshtomp, quick!" Ash said. "Koraidon, out of flying mode and break yourself free – wait, I know! Flamethrower, keep up a screen between yourself and Lunatone!"
"Yah!" Marshtomp shouted, breaking back out from underground and hitting Lunatone with his Dig attack, then followed that with a Muddy Water as the Meteorite Pokémon ascended back into the air.
"Psychic," Tate said.
Solrock pushed energy at Koraidon, making her wince.
Was she a Fighting type, then?
"Flamethrower and get close!" Ash called.
Flames flashed over Lunatone's rocky body, then Koraidon pulsed her sunlight on for just a moment as she slammed another Rock Smash into Liza's Pokémon.
"Reflect!" Liza realized, prompting her Lunatone to protect both Pokemon. "Now – Moonblast!"
"Dodge!" Ash called, and Koraidon sprinted for the far side of the gym – far enough to avoid some of the blast of lunar light. "Now, uh… sunlight on and use it to sprint in against Lunatone!"
"Solarbeam!" Tate ordered. "Get that Marshtomp!"
"Koraidon!" Ash called, waited a moment until Koraidon was in position, then revealed his plan. "Shield Marshtomp with Lunatone!"
"What?" Liza asked. "Tate, wait-"
It was too late. Koraidon grabbed Lunatone and hurled the Rock-type in between their teammates, meaning that the boosted Solarbeam just slammed right into Lunatone, and the impact was enough to knock them out of the fight.
"Cut out the sun!" Ash said. "Great work, guys, now let's beat Solrock!"
"That was an intense battle," Tate said.
"Yeah, you've got a good hang of how your Pokémon can co-operate in battle," Liza agreed. "It looked like you were in trouble in the middle there?"
"Yeah, we actually don't know for sure what types Koraidon is," Ash admitted. "Though after that I think she must be Fighting-Dragon, with how hard Moonblast hit her compared to everything else."
"I do not think I like moonlight very much," Koraidon mumbled. "But I will do my best to get over it."
"You did great, Koraidon," Ash told her.
Tate and Liza exchanged a glance, then spoke in unison. "You've earned the Mind Badge!"
They held it out, and Ash reached out a hoof.
The expected colour-change happened, his ring going a sort of reddish purple-pink, and everyone from Ash's travelling group looked expectant.
"The Original One breathed alone before the universe came," Ash recited. "...huh."
"What just happened?" Tate asked, very confused.
"I just thought," Ash tried to explain. "Dad must have been really lonely… maybe he still is. Maybe that's why… you know. Mom."
"I actually think that's the opposite of an answer, because now we're more confused," Liza said.
AN:
That's one way to find out what type your Pokémon is, if you don't know.
Not conclusive, but a good clue.