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O (0)

Main character
Japanese Name オー
Light Novel Utsuro no Hako to Zero no Maria


O, commonly misread as '0' (zero), is the antagonist of the series and the giver of 'Boxes' that 'grant wishes'. O has a strange curiosity with Kazuki Hoshino which is the source of much of the mayhem within the story.

Source: Hakomari Wiki