Fai underestimated Kurogane so bad. He really, really thought that “Good morning, Kurogane” and one arc of being a dick would be enough to push Kurogane away. To make him not care.
Because not only he was super surprised when Kuro-tan just cut off his arm to save Fai’s sorry ass, but also the whole Ashura-o thing. Im pretty sure during their little fight, Fai tought Kurogane would kill him.