Gladion is actually half-prepared for Diana, which is more than what most people get.
This battle is probably the funniest thing in the story so far. The editor might put it on the TV Tropes page herself.
Diana and Eli left the ranch as two kids on a trainer's journey. After winning a double battle against small twins, they found themselves as two kids in the middle of a mystery.
It started when they ran into Hau. Hau, it seemed, had chosen to camp outside instead of going back to the Pokémon Center, and had encountered an angry boy who looked just a bit older than the three Melemele trainers, perhaps twelve or thirteen. This boy was tall, blond, and emo, dressed entirely in slashed black clothes. Despite this, and despite Diana's very clear interest in a boy who was not any of those things, she was still distracted.
Fortunately, Ilima was the only one to short-circuit her brain, which allowed her to respond when Eli questioned it.
"This guy has green eyes," she explained. "Green eyes are pretty. I got distracted by Lillie before I realized how horrible her Pokémon training skills were."
The conversation was interrupted by Hau. "I guess I lost again," he said, not sounding at all like he cared.
The blond boy seemed to get angrier at Hau, just for taking a loss so calmly. "Are you even taking this seriously?"
"I'm seriously enjoying myself!"
Mystery boy didn't seem to know what to do with Hau. Diana could relate. "Need help, Hau?" she asked, pushing aside her conflicted feelings about the sudden appearance of cute guys in her life. She was sure that she'd settle for science again once she finished puberty.
"You missed it, Diana!" He gestured to the angry boy, who suddenly looked a bit less angry. "This guy beat me! He had some kind of Pokémon -"
"You're Diana?" the angry boy interrupted. He was more surprised than angry now, though. Interesting.
"That's me," she confirmed, looking back at him in confusion. "Why? How do you know my name?"
He ignored her questions. "You're not very cautious, are you? Some Pokémon have a better sense of self-preservation than you do."
Diana tilted her head in acknowledgement. "Yeah, I get that a lot." Behind her, Hau and Eli both nodded their agreement. "But you never answered my questions. Who are you, and how do you know my name?"
The boy looked at Hau, then back at Diana. "I'm Gladion. I know your name because I know how the internet works - you apparently did an entire trial with only a Rowlet."
"Someone posted that?" Diana asked, surprised.
Rotom laughed from her bag. "You said you wanted your parents to see your adventures," he reminded her. "I might have started a YouTube channel for you."
Gladion didn't notice her talking bag. Or maybe he did and didn't care. "I battle for my partner, Null, to make it strong. This includes taking jobs for Team Skull, which is where I heard your name for the first time."
"You work for Team Skull?" Eli interruted. "No wonder you need to make your Pokémon strong."
Diana immediately fist-bumped him.
Gladion's hand twitched, but he didn't respond to the insult. "Battle me, Diana! Show me how you managed to pull that trick off!"
And before she could say anything, he sent out a Zubat. Rockruff did manage to 2-hit KO that one.
Gladion's second Pokémon was nothing like the first.
Gladion called it Type: Null, and that was the only confirmation they had about its identity. Diana had never seen it before, and even Rotom had hit a blue screen when he tried to look it up. The beast walked on four legs, but two of them appeared to be bird talons while the others were paws. It had a webbed tail. A mask covered its face.
And Diana still practically threw herself at this monster, screaming "Puppy!"
"Not a puppy!" Eli complained. His hand was twitching now, and Gladion briefly wondered why he could smell smoke. "That is definitely not a puppy!"
"We were in the middle of a battle!" Gladion added.
Diana's response was to ignore them both, holding out her hand for the strange new Pokémon. "Don't be afraid," she told it, and it growled but observed. "I just want to see what you are. You can understand, right? I'm just curious."
The Pokémon stopped growling, looking at the girl in suspicion. Then, surprising them all, it gently butted her outstretched hand.
Gladion's scream of rage pierced the sky. "You can't just pet the enemy!" No one was sure if he was talking to Diana or Null. Not even Gladion. But he quickly went into talking to Null, if he hadn't been already. "Bite her! Take her hand off!"
That got her attention. "Excuse you!" Diana stood up, and Null made a sound that was either another growl or actual laughter. "Do you want me to call the cops on you? Because all I wanted was to get a little bit of knowledge, and you tried to get your Pokémon to attack a trainer. Don't you know how science works?"
Hau stepped back. Hala getting angry had scared him once, and now Diana was quickly following the same path. Gladion, on the other hand, didn't even flinch.
"Don't you know how battles work?" he shot right back. "You don't walk into the middle of a battlefield! Even your Rockruff knows that's a bad idea! And even if it wasn't a battle, you don't pet random dogs you see on the street!"
Diana looked over her shoulder, where Rockruff was staring at her in open disbelief. Then she looked at Eli, who still hadn't stopped shaking. Then it sunk in. "Whoops."
"Whoops," Gladion repeated, mockingly. "You jumped at a Pokémon belonging to a Team Skull enforcer, and you didn't think I'd call for an illegal move?" He glared at Null. "And you! You not only let her do it, you played along! I thought we agreed that it was a bad idea!"
"I apologize," Null said, and Rotom and Rockruff both flinched - even the voice sounded unnatural. "She did not seem to be a scientist. I shall follow your orders more closely, Master Gladion."
"Master?" Rockruff repeated.
"Of course," Null confirmed. "Is the girl not your master? Do you not listen to her commands in battle?"
"I guess she is my master, if you put it that way," Rockruff agreed. "But we don't call her Master Diana! We just use her name!"
"I suppose each Pokémon has a different way of doing things. Zubat calls Master Gladion 'banana head,' for some reason."
As Rockruff fell over laughing, Null took the opportunity to take him out. Then came the Murkrow of retaliation, and then Gladion lost.
Gladion stared at Null's ball for a while, as if its failure was something on his part instead of Diana being a better trainer. "It looks like I'm still not ready," he told it.
"Ready for what?" Diana interrupted, and Gladion turned his full angry stare on her.
"Never mind!" he snapped. "Your friend here is strong, but not serious enough. You're literally insane, and let's not even start on what's wrong with Eli!" He took a moment to calm himself down. "You're strong," he said again. "I can say that much. I'll challenge you again when one of you grows a brain cell."
He turned to leave, but froze before he could take a step. "Oh, crap," he muttered, and the natural question the others formed was answered before it was asked as two Team Skull grunts, one male and one female, came aggressively dancing up to the four kids.
"Aww!" the female grunt cooed. "Looks like we crashed a play date! Who are your friends, Gladion?"
"They're not my friends," Gladion said through his teeth.
"And here we were, thinking you'd finally proven yourself worthy of Team Skull," the male grunt snorted. "Let me guess. Couldn't even beat the one in the yellow shorts, am I right?"
"Oh, he beat me," Hau confirmed. "It's Diana who gave him trouble!"
"Then let me!" said the grunt, pushing past Gladion to face Diana himself.
"Leave them alone," Gladion interrupted. He didn't sound like he was defending them. It sounded like he was bored. "If I couldn't beat the girl, and you can't beat me, imagine what she'd do to you." He leaned in close to the grunts. "Your Pokémon will get nuked."
"You owe us after wrecking our chances at stealing Brooklet Hill's Totem," the female objected.
"I owe you nothing. You said yourself I'm just hired help, right?" He shrugged again. "If I'm not part of Team Skull, that means I can do what I want. And I will stop you from getting involved with these kids."
The grunts had a short conversation, and then decided that they did, in fact, believe Gladion had a point when it came to going up against them. The Skull grunts left, and Diana sighed in relief.
"I guess I should say thank you?" she said uncertainly, but Gladion just turned away.
"You're not the first one, Buttercup." It was clear that he meant it as an insult, but the spontaneous nickname still took her by surprise. "Unless you mean to thank me for saving you from your own stupidity."
"Buttercup?" Eli mouthed, not daring to raise his voice to even a whisper despite his desire for an explanation.
Diana was confused, too, but she pushed it aside for a bit. "So, since I defeated it, can you give me some idea of what exactly that Pokémon even was? It was cute, and strong, but -"
"I don't have to tell you anything."
"Not even why you regularly go around stopping Team Skull from challenging people while you work for them?"
"Not even that." Gladion turned his head back, briefly, before starting away. "I'll see you later. Be ready."
And he was gone.
"Well, that happened," was all Diana had to say on the matter. But now that the weird part had passed, her travel companions were coming back to their senses.
"Why did you jump at a strange Pokémon?" Eli demanded. "What possible scientific knowledge could be gained from it? What purpose did it serve?"
"I could have figured out what it was," Diana explained calmly.
Eli facepalmed. "I was worried! Don't do stuff like that anymore, ok? If Gladion hadn't had the sanity you didn't..."
"He really knew how to deal with Diana, didn't he?" Hau laughed and crossed his arms behind his head. "I think I might ship them now! He can keep her impulses under control, and she can teach him how to not be a total poop."
"But she's clearly into Ilima!" Rotom protested.
"I'm with Hau," Eli interrupted. "Ilima's got Diana's weak points covered, but he's obviously a little bit scared of her. Gladion didn't even flinch when she snapped at him, and he snapped back. And there's the nickname thing - all it needs is a little twist, and it can easily be used in a romantic context."
"But that was more of an insult than a nickname," Rotom pointed out. "Even a friendly one. She's wearing a flower print shirt, had a flower hat in the video he knew her from, and looks vaguely like the Powerpuff Girl. Besides, Ilima is obviously supposed to be her love interest!"
"We're all real people here!" Diana protested. "All of us! Yes, both of them are cute boys, and yes, I am a mess. But you should stop shipping real people! That's what that nutcase in Kalos did, right? The Team Flare leader? Do you want to blow up the world?" They all agreed that they didn't. "Good. So shut up and let me handle my love life - or lack of one."
Besides, she had more important things to worry about than what boy she liked more. Why Gladion knew Eli, for example - he hadn't been in her trial video. No one had even mentioned his name.