I think Kiri is a Kamaitachi, a yokai that's quick, nimble, with sickle like claws/protrusions that kill people without them even knowing through extremely sharp but painless wounds (dead before they even realize it). They're usually depicted as weasels but some of them are depicted as Mantises, like Kiri here.
Design is so sick I still would in that form. Joking aside I kinda hate when mantis' scythe/claw is depicted as bladed weapon in most manga when in reality it's used to grab prey.