Shiro is on the spaceship because his sister.
Shiro mentioned he had a sister he hadn't seen in a while, who's place he might have taken on the ship due to his androgynous appearance, explains why he ended up on the girl's side of the ship. Either that, or he is there to cause her despair if he should die much like Ayano Kirigiri, who claimed she would feel despair if Kira Iwata should die. Etsuko and Emilynn know each other.
Unlike its sister, the characters of the Despair in Our Stars don't appear to know each other. However, Emilynn and Etsuko appear to know each other prior to the story since Emilynn takes Etsuko somewhere to talk and Etsuko calls her Emily. Due to the infamy surrounding Emilynn's name and somehow being accepted to Hope's Peak Academy despite this, Emilynn and Etsuko may have met since Etsuko is a psychologist, making it reasonable to assume Emilynn was one of her patients, or is still one. Either Yuuki or Mei will be the first victim/killer or Etsuko or Emilynn will.
Due to how much Yuuki and Mei know about Akita, one of them are likely to die.- For Yuuki to die, the gender separated spaceship motive and occupying the wall of the gymnasium may have made it more likely that someone decides to kill them due to their position.
- For Mei to die, her relationship to Akita made her and obvious death in her story despite Akita's death.
With Etsuko and Emilynn, they appear to be some of the few characters who know one another, making it likely that one survives to explain the relationship at some point after the other dies.
- For Etsuko to die, the mystery behind Emilynn's infamy and repeated talent would take importance over Etsuko's apparent happiness of being involved in a killing game.
- For Emilynn to die, her close encounters with death eventually take her out.
