Zionel’s answer to ““once a gun appears in a story, it has to be fired.” He repeats this many times and in the end Aoma…” > Likes and Comments

If you notice lot of deaths have been by strangulation in this story.
Kumi Adachi,Ushikawa,Tengo's mother

I think Kumi is more related to Tengo's mom than Ushikawa since she had some personal liking for his dad as well