i'm not sure if i'm using the correct word to describe this, but i'm looking for a heroine that is usually quiet but hides that she is actually very scary, strong, strong willed, or sadistic.
preferably not yaoi, yuri, smut, mature, ecchi, shounen ai, shoujo ai, or horror..

thanks a lot for the help in advance 🙂

Akuma to Love Song
I don't think you mean actively hiding, but more like no one knows, because she is quiet right?

in need of romance?
uncommon, sad, sweet

Can't really say that they are quiet, but they are strong and hiding it.
So maybe these?

W change!!
Boy of the female wolf (manhwa)
Fly high!

（・◇・）/~~~ ♥

Yamato Nadeshiko Shichihenge ~Socially Awkward at school, but at home or events can be quite scary & sadistic...sometimes. Eventually both worlds intermingle.

Last Game~Quiet school life, but is very strong-willed/strong.

M to N no Shouzou~Quiet & shy...then very scary/sadistic when something violent happens to her.

Um.....maybe...
A tackle on my life
And kinda
Hes dedicated to roses
? I m not sure...But I guess these kinda work 🙂

I don't know about sadistic and scary. But there is non overreacting but strong willed heroines:
Akagami no Shirayukihime
Ouran High School Host Club
Yamada-kun to 7-nin no Majo
Mugen Spiral
Otoyomegatari...kinda
Koi dano Ai dano...kinda fits your bill, she's manipulative and likes being the puppeteer
hope it helps.

Kujaku no Kyoushitsu