Straight: Masako is a genuinely upbeat and kind-hearted girl... She also loves beating up the bullies that constantly harass her. And she does with a smile on her face. She also loves to make terrifying threats to Kenji.
Exaggerated: Most of time Masako acts like she was purity incarnated... But whenever she flips, she is so insane, that no one would even dare to go near her, not even the most detached Cloud Cuckoo Lander.
Downplayed: Type B Tsundere, with her violent side exaggerated and played for laughs.
Double Subverted: ...Which is a facade, revealing her to be The Ophelia.
Parodied: Masako runs around, carrying large and dangerous weapons and uses them to relieve her frustrations upon Kenji in a slapstick way. Masako also pops up near Kenji with a Nightmare Face every time, scaring the piss out of him.
Masako is Ax-Crazy, because years of constant abuse from her family and friends has chipped away at her sanity and she can no longer bear the emotional pain.
Masako is a nice person, but the stress inside her boils and she finally snaps and decides to hurt her loved ones.
Masako's friends can't handle her psychotic episodes and either gradually fall away out of fear or frustration or snap back at her after a particularly bad episode.
Masako's friends and family don't recognize her problems because she seems too cute to have serious issues. As a result, she doesn't get the psychological help she needs and ends up hurting someone.
Masako channels her anger, stress and pain by going insane and physically and verbally lash out at her tormentors, justifying her actions. Between periods of Pay Evil unto Evil she's as cute and friendly as can be.
Masako endures it and remains friendly but still manages to creep her friends and family out with her eerily calm behavior and occasionally crazy actions.
Masako and her friends eventually decide to stay and help her - if eliminating the psychotic side isn't plausible, then redirect it towards the bad guys or something.
Oniichan, please go back to Cute and Psycho... Or I'LL RIP YOUR FUCKING FACE OFF!!!