Mahiru Banba

A rather shy and mysterious-looking young woman who's a former assassin who's developed a second personality at some point in her past named Shin'ya Banba who's a quite more rude and violent personality than the actual Mahiru. Both are like two souls who share the same body, thus both never appear at the same moment throughout the plot, yet both are able to communicate with each other to a certain degree.

Mahiru is a shy and timid girl who often has trouble speaking for herself. She is easily subject to bullying, and is often defenseless. However, when the sun sets, Mahiru's alternate personality, Shin'ya Banba, appears. Shin'ya is the complete opposite of Mahiru, and refers to herself (or even himself) as "ore (俺)", a masculine term. In contrast to her host personality, Shin'ya has an aggressive and borderline insane appearance cultivated to protect Mahiru. Shin'ya is able to enter a temporary state that grants her superhuman strength, though she is unable to use it for extended amounts to time without rest. She also seemed to be excellent at tracking her targets

(Source: Akuma no Riddle Wiki)