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Tear Jerker: Knite
Sen's abusive step-mother who just wants to be an actual mother but has constant miscarriages due to the poisoned water.Which is causing Kai's sister's sickness, and is the fault of Kai's father's corrupt practices.
Min-Min: "Could they be the real stars?" Kai: "No, Min-min. They are simply delusions of hope. Now don't move so much or you'll pull out your IV." Min-Min: "What does "delusion" mean?" Kai: "It's something that you should hold on to."
— Chapter 3 bonus
The reason why Kai went to Sen's house in Chapter 3 and everyone knows how that ended...
"... I was hoping we could hang out one last time. Too bad that went horribly wrong...." - Kai
In the chapter 4 omake Kai looking back on the friends he made ... and then swearing to return to make things right.