[Haikyuu!!] Brother-Con Simulator

1.

Question: You wake up one morning and realize you've transmigrated. What do you do?

Answer: (swearing) Why did I transmigrate as a twin, and what the heck is this trash System?

Mission objectives:

Unlock your skill points: 0/100

Take the fall for your older brother: 0/?

Keep yourself chaste for your older brother: 0/1

Sakusa Seiya: ...

Is something wrong with you? What do you mean, keep myself chaste for my older brother? Am I going to have to fall in love with my twin in the end?

The System shrugs helplessly: This is the Brother-Con Simulator, after all.

2.

What does it feel like to wake up and find your twin has vanished from the world?

Sakusa Kiyoomi: Thanks for asking. It's terrifying.

One day, his twin suddenly grits her teeth and says something completely incomprehensible to him.

"Goodbye, my difficult 'older brother.'"

After saying it, she passes out. When he wakes again, it's the next morning, and he's in his own bed.

At first, nothing seems different. Half an hour later, he realizes that every single one of Sakusa Seiya's belongings has disappeared from the house.

Sakusa Kiyoomi calls Komori, who asks him who Seiya is.

His parents give him the same answer when he calls them. No one but him remembers Sakusa Seiya. It's as though he never had a twin sister at all.

What used to be an everyday-life manga has become a suspense thriller overnight.

It isn't until after he joins BJ, a few days before their first match against AD since Hinata Shoyo's return, that one of his teammates points to a magazine and enthusiastically discusses the long-haired beauty with blue highlights on the cover, saying she looks a lot like Kiyoomi.

Sakusa Kiyoomi leans over to look and freezes on the spot, nearly crushing the cup in his hand.

* Includes sibling affection.

* The timeline is jumbled, so please keep track carefully.

* Not sure yet whether there'll be romance, so it's tagged as no CP for now, though there will be some ambiguity (wiggles fingers).

* Cover art thanks to my good friend @Mu Huai.

Theme: No particular theme yet.