Bungo Stray Dogs season 4 episode 9 revealed one of the biggest mysteries of the series.

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The episode unveiled Yosano Akiko's backstory and her connection to Mori.

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She was previously only referred to as the "angel of death."

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Season 4 episode 9 was titled, "To Dream of a Butterfly."

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It opened with Tanizaki and Kenji asking Yosano about her connection to Mori.

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Yosano, not wanting to reveal her relationship with Mori and Fukuzawa.

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Mori recruited Yosano to the army due to her ability to heal people in a war.

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While Yosano tried to put an end to the painful cycle, Mori forced her to heal the troops.

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Nevertheless, Fukuzawa tried to prevent it, and thus stopped his partnership with Mori.

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Nevertheless, it is yet to be seen if Francis has laid a trap for the agency or not.

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