One day in a noble house from the Soul Society, a pair of twins were born. Both were born with high Spiritual Pressure, and were being trained since birth to become strong. However one of the twins were far more cunning than the other, and the remainder was far too naive.
And then one faithful day, they both ventured outside the Seireitei into the Rukon district without informing anyone.
As they both explored a nearby mountain, they came across a child the same age as them, heavily injured. One of his legs were broken so he couldn't walk by himself, and he also had a concussion, so he was sure to die.
The two brothers knew a little healing Kido, which wasn't enough to fix his leg, but it would keep him alive until they found help.
"Shiriyu, look at how pitiful it is. How about we lessen its pain?", one of the brothers said.
"What do you mean?", he asked nervously.
Jinriyu smiled as they pointed to a heavy rock nearby. He asked Shiriyu to bring it to him, which he did without hesitation. Jinriyu picked up the rock with both hands and raised it above the child's head. Just before he dropped it, the child suddenly woke up.
"Why?"
Those were the only words he could utter before Jinriyu dropped the rock on him, crushing his skull. Those words haunted Shiriryu as he made an expression of horror. The blood of the child seeped out of the crevasses as his spiritual pressure disappeared.
"Brother? Why did you kill him? I'm going to tell father-"
He slowly tried to back away, but then Jinriyu quickly grabbed his hand, stopping him from leaving.
"You just gave me a murder weapon to help me kill it. That makes you an accomplice. If father finds out, then you will get in trouble too. As long as you listen to me, then you won't have to be worried about that."
With the burden of hesitation, Shiriyu began to dig a hole using Kido. Jinriyu threw the body as well as the rock inside the hole, before quickly filling it back up. They cleaned up any left over blood and started to make their way back to the Seireitei.
"But why did you kill him?", Shiriyu asked.
"Why do I need a reason to crush an ant which is lower than me? It was stinking so someone would've had to clean it up anyway."
"But what if we get caught? Someone will find them eventually.", still unsure of his actions, he continued to pester his brother.
"Just shut up and listen to me. As long as you do as I say, then that will never happen."
From that day on, Shiriyu would prioritise his brothers needs above his own. If he wanted something he would get it, if he needed someone gone then he would kill them, as long as he listened to him, he didn't need to worry.
But he only realized that he was being used the moment he was thrown away that day. He was no more of an ant just like the innocent child that he helped to cover up.
Why did Jinriyu abandon his own brother? The answer is simple really. He found a new toy which he wanted to corrupt in his image.
Shido kept trying to sleep in his dorms bedroom, but he felt rather uncomfortable because of the person who he was paired up with. He rolled over only to see Jinriyu who was sitting in his own bed, just starring at him.
He got creeped out, and quickly rolled over as he pretended to not make eye contact with him.
"It's rude to pretend not to see someone. I haven't been able to sleep since coming here, so make some conversation with me."
Shido did not want to associate himself with him, especially after what he did to his brother. He considered pretending to go to the bathroom, and escaping this situation but he was sure that Jinriyu would just bring it up again. He turned over while sweating bullets, as he nervously said,
"You haven't been coming to classes in a while? Your in class 1 too aren't you?"
"You are correct. I don't have a slight interest in going there, as I have better things to be doing."
Shido was unsure how to respond, but he carried on with the conversation anyway.
"So do you plan on entering the Tekken event? You would be able to become a Soul Reaper much faster if you did."
"I'll enter the event to satiate my boredom, but becoming a soul reaper doesn't interest me in the slightest. I have no reason to join that group of lying fools."
Confused, Shido then asked, "So why did you even join the academy?"
He began to divulge incredibly sensitive information which could warrant his execution if leaked to the public. He even exposed his knowledge on Shido's past which he had not told Kai yet.
Sensing danger in this mans words, Shido got up out of bed and prepared to unsheathe his Zanpakutō. For a brief moment he considered killing Jinriyu right there and then.
A dark shadow covered Shido's face as he asked Jinriyu with a voice of pure malice and hatred,
"How do you know about that? Were you there that day? If so then I should kill you all the more."
He made a sly smile,
"That inner rage which you've been hiding inside you all this time. Not even your friends know about this side of you? Well I was able to see it at a glance. As it turn out our goals align pretty well. Would you like to hear this scheme of mine? It would benefit both of us greatly."
He fully unsheathed his sword and was just about to cut Jinriyu's head off. But stopped just at the base of his neck. Surprisingly, Jinriyu didn't flinch in the slightest.
While exuding an aura of absolute confidence he said, "That's not very nice. I wouldn't like killing my only ally right here right now."
"Since when was I your ally?"
"Ever since you helped me at that entrance exam. It didn't matter if you admitted to it or not, since the very beginning you have always been my ally."
Shido sheathed back his Zanpakutō, before returning to his bed.
"Now that you know I'm aware of your secret, are you going to help me?"
Shido heard him, but decided to ignore him for the rest of the night. He was unsure on how he viewed that Jinriyu. He was a manipulative bastard who he should keep away from at all costs, but on the other hand, he was the only one who could help him right now.