Shoujo Pika Ichi by MAKI Youko and MOCHIDA Aki

[Kaiten]

Pika Ichi
Written By: MAKI Youko
Art By: MOCHIDA Aki
Genre: Delinquent, School Life
Serialized In: AriA
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"Pika Ichi" is the story of two painfully shy teens who love yakuza movies. An incident at school pushes them over the edge, and they decide to drop their mousiness in order to dispense some "justice."

-from the Omari's Sister website

By the mangaka of Aishiteruze Baby