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.
If anything, Kai, Min, and Sen's situations in general. Sen was abandoned by his mother and has to live with an abusive and mentally unhinged stepmother and Kai and Min's father is a corrupt politician who has done very little to protect them and, in the end, Kai is sent away when his father becomes apart of a scandal, however, Min (who's terminally ill) has to stay behind and it is subtly implied that she'll succumb to her illness in the near future, probably before Kai may even return.
The latter cases are even more heartbreaking, if you consider that Kai and Min seem to be serving as Yuumei's avatars.