Maybe she's cosplaying as Mitsuru Ushijima, a japanese general during the World War II, who commanded the 32nd Army during the Battle of Okinawa. The battle eventually ended with the defeat of Japan and the allied victory; due to this, the general commited suicide doing a harakiri.
There's a war movie called Hacksaw Ridge, which focuses in the experiences lived by Desmond Doss in the aforementioned Battle of Okinawa. The thing is, in the ending of the movie, they recreated the general's suicide. (WARNING: Gore, spoilers of the movie.) As you saw in the clip I shared, the general is wearing a white-buttoned and baggy shirt, and he commits suicide with a short knife which was sheathed in a leather sheat. The similarities between the movie scene and this drawing are really clear.
Considering Khyle. likes war movies (I say this based in post #7288695) I think perhaps he was trying to make a reference to Hacksaw Ridge and the general's death scene.
i may be wrong, but i think what the knife and the board is for is finger shortening (yubitsume). as for who she's cosplaying at (if she is cosplaying at all) i'm not sure, may be some obscure movie reference.