It makes no sense that Shizuka just submits to whatever torment Marina puts her through. Even when they are alone 1 v 1 she just takes it like a bitch.
I can only assume it's the guilt she feels from taking Marina's father away that keeps her subservient. Probably because she also lives in a fatherless family.
And it's the moment that Takopi kills Marina when Shizuka can finally abandon her guilt and subconscious. The current Shizuka is now someone with nothing to lose and nothing holding her back