Pinkie was just like any other high school aged girl. She was a pastel shade of pink, liked funny words, and was pretty darn sure that she was in a tv show, movie or some form of poorly written fan fiction. Although she did sometimes suspect that last one wasn't entirely normal. And she was leaning towards the last option. I mean, just look at the way the last two sentences started. Pinkie's high school life was fairly uneventful until she showed up.
Her name was Marie-Sue, and she was inserted into Canterlot High super abruptly. She scanned the class with what Pinkie assumed was meant to be a smile while Miss Cheerilee introduced her.
“And she's from... Actually the memo I got didn't have a terribly specific back story for you; where are you from?”
“I, uuh,” she broke from her weird half smile to turn to Cheerilee, confused “Canterlot, I guess, that's where the princesses live.”
“Oh, I... Ok. You can take that free desk over there.”
As Marie-Sue walked slowly to the desk in front of Pinkie, Pinkie wondered if she was related to their Principal. She had that same weird leg thing going on. It was probably genetic. It had to be, surely, both Princessipal Celestia and Vice Principal Luna had it, and now Marie Sue too, who did look a little like the two of them in the face, as well as the legs now that Pinkie though about it. Good thing it only seemed to affect the legs and not the torso, Pinkie hated having to strain her neck to see past the people in front of her. Although it did seem to her that when she did need to strain, her neck could extend just a little further than should be physically possible.
Miss Cheerilee had begun her lessons for the day, but Pinkie was still contemplating the new arrival, as, it seemed, was most of the rest of the class. She was a light shade of blueish-grey, with a long flowing mane of red and blue that somewhat clashed with the rest of her aesthetic, but might seem cool to some people, Pinkie supposed.
After some time Pinkie realised that Cheerilee hadn't really said anything for a while. She was just standing there with her eyes closed, breathing slowly but deeply. Why hadn't anyone said anything? Had they even noticed? Pinkie looked around to find that everyone was still transfixed on the newcomer. Marie-Sue hadn't seemed to have noticed either, though her preoccupation seemed to be with the state of her nails.
“Miss Cheerilee, are... you okay?” ventured Pinkie Pie.
“Yesss” Cheerilee replied in a soft voice that wasn't quite her own.
“Oh. Shit” said Marie-Sue under her breath.
Good. Someone else had finally noticed.
“She doesn't normal do this, does she?”
Marie-Sue was addressing the whole room, but other than Pinkie, no one was listening.
“No. This is weird.”
“Shiiit.” said Marie-Sue again. It felt a little out of place but Pinkie figured she may as well go along with it. “She's been replaced with a Changling. We have to take care of her.”
“What's a Changling?” asked Pinkie, thinking to herself how convenient her knowledge gaps would be for readers of the story.
“They can assume the form of anyone, then they feed off any affection around them.”
“That doesn't seem entirely correct, but-”
“Don't over think it!” Marie-Sue snapped, “She must be picking up on the adoration that your classmates have for me, I am very cool and pretty. Did you know that Princess Celestia is my aunt? Catching the Changling was the whole reason I was sent to this school”
“I knew it! What's up with your legs?”
“What?”
“What?”
“I... Just. Whatever” Marie-Sue shook her head a little before starting towards the front of the class.
Luckily enough the Cheerilee shaped creature hadn't moved while they were discussing her.
“You're going down, scum.” said Marie-Sue, as she approached the Changling, pulling an impossibly long sword from her handbag.
A sword. Huh. It didn't really feel in-universe to Pinkie.
“You'll never catch me!” screeched the rapidly darkening... thing, it's voice now sounding nothing like Cheerilee's.
The Changling dodged towards the windows as Marie-Sue swung her obscenely large sword. Marie-Sue pulled it back, but the Changling had made it to the window already and was climbing through.
“You will not ever catch me!” it yelled as it fell.
“Yes I will.” called Marie-Sue after it.
“No,” it called back, now running across the carpark “you wont!”
“I will!”
“She can't hear you any more,” said Pinkie, who was now standing beside Marie-Sue at the window, “She's gone.”
“For now, but it'll be back, and in greater numbers.”
“Really?”
“No, not really. But people love pop culture references.”
“Oh.”
Pinkie had no idea what that meant.
“I need to talk to my aunt.” and with that she left Pinkie with the slowly awakening class.
Later that night while Pinkie was trying to make sense of the day's events, she received a message. Hmm, she didn't recognise the number. She opened it.
“This is Marie-Sue. We found Cheerilee, but Celestia wants me to stay at Canterlot High until I can catch the Changling. For some reason you seem to be able to keep a clear head around me, I might need your help throughout the year. Hope you're okay with that.”
Oh goodness, thought Pinkie, this is gonna be my year now.
A spin off.
Shit.
The glass was back, but no one noticed it.
Marie-Sue was out for the day again, which meant the class was able to function normally. They'd mostly gotten used to her by now, but were still unable to run at one hundred percent in her presence. Probably never would. Pinkie didn't know why she was able to resist Marie-Sue's effects, but guessed it was related to whatever it was that allowed her to see the glass. Pinkie was convinced that the others would be able to see it, if only they'd look. They never did. There was always something to draw your attention away when the glass was present.
This time it had replaced the entire left wall of the classroom.
Pinkie wondered what the glass was doing here this time; nothing interesting was happening, they were just taking notes. Maybe she'd be able to get someone else to see it now.
"Psst, AJ, it's back."
"Gimme a sec, I'm a little behind the teacher."
"No, it might not... I can't ever tell how long it'll last."
"In a sec, I said."
Fine.
Pinkie stared into the opaque mist behind it, wondering what was there. Sometimes when she stared into it, she could feel it watching her back.
Then, suddenly, it was gone.
"Alright, let's see this... Damn it, Pinkie, I told you there aint no mysterious glass."
"No, I swear, it was just..."
There.
It was just there.
Had it disappeared because Applejack was about to look up, or had something been keeping her from looking up while it was there?
As Pinkie tried to rejoin the note taking she wondered if Marie-Sue would be able to look.
Be able to see.