So I found this after hitting the I'm Feeling Lucky button on the Gyaru tag, and after reading up to here I gotta tell y'all that I really don't think I was very lucky at all.
It is technically possible, there's a rare medical condition called precocious puberty where girls as young as five have started having their period as a result.
I think she might be Yandere with considering the obsession hints on page 22 but at the same time it feels a bit too much an anime trope for what has been a pretty down to earth tragedy manga.
This manga.... Got me so interested.... This is very fucked up.... Reminds me of happy sugar life..... Definitely got them elements... It has the type of plot twists like higurashi....