Surprisingly, it was Aomine who first accepted Tetsuya's existence.
Though he will deny it to his deathbed, he'd been afraid of ghosts his entire life. For this reason, Akashi had been reluctant to introduce them. But Tetsuya had other plans, scaring Aomine after hours in the gym, and after the ghost successfully hit him in the head with a basketball, Aomine declared they were best friends and that was that.
Once again, Akashi was impressed with how simple the brute was.
After that, Aomine made it a point to bring a Ouija board or the like with him in order to communicate with his new ghost friend. Before using it he always warned Tetsuya not to bring along any of his own ghost friends, to which he retorted his friends didn't want to meet him. This always resulted in a childish argument between Aomine and a seemingly invisible being that cemented the idea of him being certifiably insane in the minds of his peers.
Then came the fateful question.
"Sooo, Tetsu..." Aomine drawled. "Can you like, possess people and stuff?"
Tetsuya guided the planchette to spell out 'never tried it'. Aomine raised an eyebrow.
"Really? Why not?"
'Didn't have a test subject.'
"Test subject, huh...I guess it would be weird to use your brother as a guinea pig, huh. Plus I bet there's lots of weird stuff going on in that freaky brain of his." Aomine tapped on the planchette with a considering look. "Ya know, I'm well-known for my stupid decision-making abilities. Wanna use me as your test subject?"
Tetsuya stared at him with wide and somewhat apprehensive eyes. He only wished Aomine could see it so he could maybe understand how ridiculous he sounded.
'Are you crazy?'
Aomine snorted. "You're asking me that when you live with Akashi Seijuro? You've got some weird standards, Tetsu."
The ghost rolled his eyes and sent a chilling mist in Aomine's direction. He shivered with an indignant glare.
"Alright, alright, yeesh! Calm down with the ice!" He huffed. "Look, I'm sure it can't be that bad. Plus it would be pretty cool, yeah? Why not try it just this once while someone is willing?"
Tetsuya sighed. 'One time, that's all.'
"Yes! This is gonna be awesome."
'Aomine-kun, I'm not sure what to do here.'
"Huh. Makes sense if you've never done it before. Maybe you just kinda need to...sink in or something?"
Tetsuya shrugged without a response. He stared at his companion, concentrating to visualize his life force. It was a fierce, blazing blue, and he squinted at its brightness. Aomine's aura was jagged and angry around the edges, but smooth and gentle in the middle. Tetsuya grinned in amusement-auras never lie. Aomine's rough on the inside and sweet on the inside, there's no longer any doubt in his mind.
Tendrils of mist reached out to him, and he quietly reached forward into the angular blue waves until-
He lets out a ghostly shriek and recoils. He turns toward the door, prepared for a reprimand at the hands of his brother.
"What are you doing?" Seijuro sounds and looks genuinely confused. "And moreover, where is Tetsuya?"
Tetsuya blinks and his face morphs into one of annoyance. How dare his brother feign ignorance in his presence! He raises a hand to lecture him when he notices something peculiar.
His arm is much, much heavier than it had been before. He slowly looks down to his outstretched hand...
...and immediately launches himself to his feet with a scream.
Seijuro looks positively scandalized as 'Aomine' rubs his hands all over his body, making assertive noises every few seconds. He frowns and crosses his arms.
"Aomine, I know you have a thing for yourself, but please refrain from self pleasure in my presence."
'Aomine' pauses and turns toward Seijuro with comically wide eyes. He points at him with his mouth wide open.
If Seijuro looked scandalized before, he looks absolutely mortified now. An extremely uncharacteristic look of horror crosses his face and Tetsuya has to stop himself from laughing. Despite his best efforts a couple snickers escape as he tries desperately to compose himself.
"Um...ahem," 'Aomine' coughs. "Sorry about that. Uhh...would you believe me if I told you I was possessed?"
Seijuro's eyebrow twitches. "No."
Somewhere in the back of their mind, Aomine screams at him not to make the situation even worse than it is. He blocks him out and continues digging his metaphorical and possibly physical grave. "Well, that's the truth. I've been possessed by the devil and he's telling me to call you aniki."
Seijuro's face goes blank and Tetsuya breaks into thought-based hysterics. Aomine lets out a dull screech deep inside their shared mind and his laughing grows even louder. He wheezes and makes Aomine pose dramatically.
"So, big brother Akashi, will you accept this devil's form of mine?"
Before Seijuro can reply properly, 'Aomine' crumples to the floor in a dull and painful-looking heap. The elder Akashi stares at it with half confusion, half satisfaction, and the younger suddenly manifests directly above Aomine's fallen body, still in the dramatic pose from before. He blinks awkwardly and slowly moves his limbs into a neutral position while Seijuro stares at him in disbelief.
"Tetsu...ya?" He blinks. "That...what..."
Tetsuya stares back at him in wonder. He's never seen his brother wearing such a face, so he commits every last feature to memory while Aomine stirs beside him.
"You...stupid ghost...bastard..." Aomine wheezes. Tetsuya looks down at him with an innocent face. "Gonna...kill your dead ass again..."
"You volunteered, Aomine-kun. You cannot fault me for this."
"Bull...shit! I didn't think you'd...fuckin'...do this shit to me! What the hell!"
"You should have known better, really."
"Oh, would you just-!"
"Wait, wait, wait a moment," Seijuro interrupts, hands up. "You guys are talking to each other."
Aomine pulls himself up into a seated position and grumbles. "Yeah, of course we are, we're..."
Realization hits him, and he sharply turns to where Tetsuya hovers next to him. The ghost waves at him, and he hesitantly waves back. He looks to Seijuro, who appears just as confused as he is. He's not sure if this makes him feel better or worse, but he's definitely feeling something.
"It would seem," Tetsuya begins. "That possession has made it possible for Aomine-kun to see and hear me."
"So, you..." Seijuro's eye twitches rapidly and Tetsuya vaguely wonders if it's unhealthy. "He volunteered to be possessed by you...and now that you are separated, he can see you." Tetsuya nods and he brings his hands up to bury his face in them. "I was not even aware you were capable of possession."
His younger twin shrugs. "Neither was I. But I guess I am. Pretty neat, huh?"
"Neat, my ass..." Aomine huffs through clenched teeth. "If you were alive I would throttle you for that."
"No, you wouldn't."
He sighs. "Yeah, you're right."
Seijuro takes a deep breath and suddenly it's as if his composure was never broken at all. Tetsuya feels a little jealous. "Alright. I cannot fault Aomine personally because he was not in charge of his body, but I will be delivering a letter to his coach to give him extra laps for disturbing me."
"Hey! Damn it Tetsu way to ruin my life!"
"Don't blame him, you volunteered." Aomine slumps down and he continues. "As for you, Tetsuya...no more possession without my permission."
Tetsuya huffs out a puff of transparent smoke and crosses his arms. Aomine mutters something about unfair favoritism.
Seijuro continues to lecture them about the dangers of unknown territory, and Tetsuya allows his mind to wander. If he could do that to Aomine so easily, who's to say he couldn't do it to someone else? A plan starts to take shape. If he could possess every single one of his brother's old teammates, certain chaos would ensue, and since his twin has such a soft spot for him he'd never be truly punished, he wouldn't have to deal with anything more than a lecture afterward.
Granted, the amount of time he'd been able to hold Aomine's form was not very long, but with some good practice he's sure he could cause some serious trouble.
A grin spreads across his face. It's only a matter of who to take over next...
"Tetsuya, are you listening?"
"Ah? Oh! Yes, sorry, I just spaced out...you know, ghost attention span..."
"That isn't a thing and you know it."
"Don't be phantasm-ist."
"That...that's not a word..."
"It ought to be."
Aomine suddenly wishes he lived in another plane of existence.