Thanks for your reply.
My Japanese friend who owns this Tsuba is sure that it was made for a Christian.
Interesting.
I said Shitake, but it could be just «Kinoko» (mushroom). So you got your "KI". The sign of the cross is from the top (the mushroom = KInoko), to the down (cloud, but maybe not a cloud) then left (chyochin) and right (Hyotan).
Another thought: mushroom= earth / cloud=sky / lantern=light or fire / Hyotan=gourd=water
I don't have any other picture now but I can show you more details from the same one: