Banba, Mahiru

Mahiru is a shy and timid girl who often has troubles speaking for herself. She is easily subject to bullying, and is often defenseless.

However,

[Spoiler] When the sun sets, Mahiru's alternate personality, Banba Shinya, appears. Shinya is the complete opposite of Mahiru, and refers to herself (or even himself) as "ore (俺)", which is a rather rough masculine way when referring to oneself. Shinya has an aggressive and borderline insane appearance. In chapter 16, Mahiru mentions that Shinya is somewhat concerned about their weight. [/Spoiler]

[Spoiler] It is revealed that Shinya came through an experience where Mahiru was kidnapped by a certain man and forced to be photographed. From that, she found escape from reality by finding out about her second personality, Shinya, and eventually killed her captor. [/Spoiler]

It can also be noted that Mahiru and Shinya's names also have polar opposite meanings, with Mahiru's (真昼) being "midday" and Shinya's (真夜) being "full night". Mahiru / Shinya wears black and white socks, possibly to represent this split personality. As well, her school uniform seems to have a somewhat odd color scheme.