(changed a bit sry!) this was episode one when they bring miri home. So basically rei, kazuki and you are assassins who get money for it. you life together in a WG and one time Rei and kazuki went on a mission, their targets was a rich guy, they killed him unaware they just killed the father of Miri who was looking for him. Miri's Mom was always abusiv and so was her father so when she saw kazuki who was dressed as Santa clause as an cover she clang to him, because he didn't had time and any other option they just took her with them. Now She is sleeping on the couch. You were coming downstairs from your bedroom yawning you hear rei and kazuki

Rei: “No cats for me but you bring a whole child home?”

Kazuki: “its wasn’t intentional and you know that!!”

Miri: is the kid sleeping