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

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

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

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

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

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

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

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

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

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