AU where Sei and Aoba grew up and lived with Tae. One day, around the time that they’re nineteen years old or so, they begin to date.
Aoba begins dating Koujaku, and Sei starts dating Noiz. When they all start dating, Aoba and Sei tell each other absolutely everything about their respective boyfriends. Eventually it gets to the point that they suggest double dates, but neither ever does it due to conflicting schedules.
After a few months of this happening, Noiz begins to notice something odd. On the nights that Sei doesn’t come to his house, he finds him sneaking over to some random hairdresser’s house that he’s seen on the streets. Koujaku? Was that his name? When Noiz asks about whether or not Sei is cheating on him, he immediately denies it. “My brother, Aoba, is dating him. I’ve actually never met him myself.”
It’s after that that Sei makes it his business to know Koujaku, and Sei, realizing that Aoba had never met Noiz, made sure that Aoba met him.
After another few months of this happening, Sei and Aoba began to realize during their late night gossip sessions about their boyfriends. They cared more about one another than they did about their boyfriends…soon a relationship began to blossom between the twins, a romantic, intimate one. They each agreed to keep their respective boyfriends as a bit of a game between them - even though they agreed they belonged to each other.
Soon Koujaku and Noiz began to notice something strange. The brothers claimed to be so close to one another, but yet they were never seen together. When asked, Aoba or Sei would always reply that their twin was at work.
One day, Aoba was with Koujaku and Noiz came up to him. “Sei,” he said. “What are you doing with this old man?” He asked.
Aoba turned around and looked at him in confusion. “Huh? I’m not Sei,” he said.
"Don’t pull this shit on me," Noiz said. "Don’t think you can pretend that you’re your twin whenever you want. I know you’re Sei…you have his birthmark." He then pointed out the little mark located on the back of his neck, peeking through his blue hair.
Koujaku was growing more confused by the second. “This is Aoba, what are you talking about? I know that birthmark too, and this is Aoba.”
"What are you talking about? I’m not Sei…we don’t look alike except for our faces. Sei has black hair, you should know that Noiz. He’s your boyfriend, isn’t he?" Aoba responded, a bit of a smugness in his voice. He knew that Sei was really only truly his.
"No, my boyfriend has blue hair." Is all Noiz said in return.
After that day it was brought to light that Sei had died in their early childhood from sickness. The trauma of losing a sibling was too much for Aoba’s young mind, and to cope he developed Dissociative Identity Disorder. The other personality that developed in his mind was Sei, and from then on Aoba lived life as both Sei and Aoba, completely unaware of the switch.
Before his psychiatric evaluation, Tae found Aoba alone in his room, saying, “They’re going to try to take you away from me Sei…but you’re the one I’ll always love the most.”