I had the idea for this before any information about cooper was released so his characterization is completely my own.
It wasn't very often that Cooper made his way back to Ohio. But when he did he always made the time to (good naturedly) torture his younger brother; which would explain why he was slowly creeping up the stairs of his parent's house at 8am on a Saturday. He wanted nothing more than to jump on Blaine's sleeping form making far too much noise for a teenage boy to handle at such an early hour. So, Cooper could hardly contain his joy as he pushed open Blaine's bedroom door and spotted a lump of teenage boy underneath the blankets.
But something seemed off.
The lump of teenage boy moved enough to dislodge the covers around its head. That was not the dark curls Cooper planned on finding in his brother's bed. Cooper's blue eyes widened as he was struck with the realization: THERE IS ANOTHER BOY IN MY BROTHER'S BED.
Somewhere deep down, Cooper couldn't help but feel a sense of pride.
"Cooper?"
The elder Anderson brother turned on his heel to see Blaine standing at the end of the hallway in nothing but a pair of boxers and his hair askew. Cooper glanced back into Blaine's bedroom and back at his brother, a lewd grin spreading over his lips. Blaine groaned and shifted uncomfortable. "That's not you," Cooper said, pointing through the open doorway.
Blaine sighed and took a slow step forward. "Can you just… keep it down, please?" Blaine pleaded his eyes wide. Cooper snickered and looked back into Blaine's room. The other boy had shifted in his sleep, kicking the blankets down to just about his hips, exposing the long expanse of his pale back. Cooper couldn't help the strange noise that erupted in his throat.
"That's not just any boy in your bed, baby brother," Cooper whispered, looking back at Blaine, "That's a naked boy."
Inching closer, Blaine made a shushing sound at his brother. The last thing Blaine needed was to wake up the boy currently asleep in his bedroom. Cooper turned to face Blaine and crossed his arms over his chest. "Why, pray tell, do you have a naked boy in your bed at 8 o'clock on a Saturday morning when mom and dad are gone?" Cooper asked, amusement alight in his eyes.
Blaine groaned as he came to a stop right in front of his brother. "He's my boyfriend," Blaine whispered, glancing into the room. Cooper deflated a little, making Blaine chuckle. "Yeah, sorry to burst your strange 'my brother has random boys over' bubble," Blaine said softly. Cooper tutted at his brother and licked his lips slowly.
"How long have you two been together?"
"Over a year," Blaine said, rubbing the back of his neck.
Cooper's eyes lit up again. Blaine knew that look. He knew to be terrified of that look. Before anything else could happen, Blaine's hands were flat against Cooper's chest and he was pushing his older brother out of the way so he could run into his room and slam the door shut. He pressed his back against the wood and sighed heavily before locking the door.
The ruckus made Kurt shift in his sleep.
"BLAINE ANDERSON! YOU OPEN THIS DOOR UP RIGHT NOW," Cooper bellowed, his fists beating against the door.
Kurt groaned and rolled over onto his back, blinking slowly. He had a pleasant smile on his face, memories of the night before flashing through his mind. But he was drawn out of those thoughts by the yelling out in the hall and the door shaking on its hinges. Kurt glanced over to see Blaine leaning against the door, a pained expression on his face.
"Blaine," Kurt said cautiously, sitting up slowly, "Why are you standing like that? And why is the door shaking like some crazed animal is trying to break in?"
"Because there is, its name is Cooper."
"Your brother Cooper?"
"Yes. He saw you in bed, thought it was me, realized it wasn't and now wants to terrorize us both. He's also aware you are less than clothed, but don't worry, he didn't see anything. I'd just suggest you get dressed and… I don't know, jump out the window or something."
Kurt's eyes widened as he shot upright in bed. He scooted towards the edge of the bed and quickly found his discarded boxer-briefs from the night before. Tugging them up his legs, Kurt jumped out of bed and tried to locate his jeans. He spun in place until Blaine pointed at the floor by his dresser. "There," he said quietly.
Kurt snatched them up and tried to pull them on without falling over. But it was then both boys realized the sound in the hall and the shaking had stopped. The both stared at one another with confused expressions on their faces.
The confusion didn't last long as Blaine's bedroom door swung open, pressing Blaine against the wall beside it. Kurt gasped a little, his hands cupped over his mouth. "Are you okay?" He asked, lowering his hands. Blaine groaned but waved him off, taking a few steps closer to him. They both turned their gaze to Cooper who was standing in the doorway holding up what looked to be a bent paper clip.
"Helloooo boys," Cooper grinned, leaning against the door frame.
Kurt was now painfully aware that he was standing in the middle of his boyfriend's bedroom without a shirt and his pants unbuttoned. He crossed his arms over his chest uncomfortably. "Coop, can you please just leave us alone?" Blaine pleaded, "At least let Kurt go home and then you can interrogate me all you want."
"Kurt," Cooper repeated, his eyes finding Kurt's. He pushed himself off the door frame and walked across the room in long, smooth strides. "Cooper, Cooper Anderson," he said, holding out his hand for Kurt to shake. Kurt side-eyed Blaine but carefully shook Cooper's hand. "Kurt Hummel… and this is not how I imagined meeting my boyfriend's brother."
"Not so bad, would have been worse if you kicked that blanket any lower and I got a view of that ass that my brother undoubtedly enjoys."
"COOPER!"