The First Mission is the 17th chapter of Bungo Stray Dogs.
Tanizaki and Naomi are eating when the conversation switches to Kyouka's past. According to the police records, Kyouka was orphaned because her parents were murdered by someone.
Kyouka is given her first mission by Atsushi, which is to deliver a piece of evidence to a judge. However, Kyouka is so eager to prove herself useful to the Agency that she overdoes it and accidentally used a stun gun on the judge. Afterwards, Atsushi cheers her up with a crepe and notices that she still has her cellphone, which while it has been programmed to stop receiving calls from the Mafia, is still a strange thing to keep. When he mentions the topic of using her Demon Snow for good, she completely rejects that idea. Just then, the phone suddenly rings, and a voice orders Demon Snow to cut down Atsushi. The owner of the voice is revealed to be Kouyou Ozaki, an executive of the Port Mafia who is here to take Kyouka back. She believes that Kyouka is being lied to and will be exploited, and that she belongs in the darkness rather than the light. As Atsushi is about to attack her, he's cut down by Kouyou's ability, Golden Demon, an ability similar to Kyouka's.