Rei and Kazuki are both assasins, and they're fairly skilled at their jobs. Rei is more monotonous and quiet, he rarely says or thing, and he never goes outside. Kazuki is more expressive and fatherly, hes a lot more sympathetic than Rei is. Though they still equally have good intentions, even if Rei isn't as good as showing it as Kazuki is. They share an apartment together, though its more so Rei's apartment than it is Kazuki's. Between the two, Kazukis is more responsible.