Thanks for the chapter :D
This chapter continues the trend of raising more questions than answers. How exactly does Aru have a seemingly normal family? Why is everything denoted in cancer terms ("Onset," "tumor," "HER2," "cytokine," etc.)? I guess that is what happens when both main characters are written like black boxes.
If I was to speculate, I would say that Aru was probably an elementary schooler at the elementary school engulfed by the "tumor" in this city. We know from chapter 1 that everyone else there was "twisted and contorted into things far from human..."
There are three possibilities: (1) Aru was a normal kid back then; (2) she intrinsically has something to do with the Onset; (3) she is "compatible" with this tumor in some way. I'm inclined to believe (2), since the Onset occured the same year she was born.
I wrote all of that, just to wonder now why I am even speculating at all over 2 chapters. Brilliant.