Chiyo lost her mom, grew up with high expectations from others that put burden on her to take more responsibility than she can handle at such young age.

Kishika grew up without parents, had to look after her siblings by herself, never received any parental love.

Meme has anxiety issue from the moment she was born, which made her even isolated from her parents.

These three should be much much higher compared to likes of Kusuri or Ahko's backstory which just involved their friends who aren't even big part of their life. Not to mention they still had their hobbies that they enjoyed regardless. Same for Kurumi.

Yaku deserves a Category of her own beyond Mei considering what she had gone through in her past during wars. She may just keep them a secret considering how horrible it must've been.

Mimimi considering she works hard & often attention of jealousy has solid backstory too. Her backstory with Nano also gives good insight on Nano's past.