Ranko Tsukigime is the protagonist of Short Peace: Ranko Tsukigime's Longest Day. She is the daughter of the owner of the world's largest parking garage company, and is somewhat frustrated with the reputation this brings her. Although she is a normal girl during the day, at night she works as an assassin, receiving missions from Fukamachi. Although she normally keeps it covered with an eyepatch, her right eye is glowing and red. It may possibly be the same as her sister Yuriko Tsukigime's third eye.

Secretly, she is something known as the "Key of Babylon", which can open the Gate of Binding according to a document known as the Yakumo Manifesto. Her true form as the Key of Babylon is a flower-like amalgamation of Japanese pop and youth culture; after unlocking the Gate, she reverts to her original form.

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