Eve_Mousey
I think that Kumagoro is a gift that Ryu's mom or sister or someone related to him gave him a longgggg time ago that he doesnt see anymore, and he carries him around for memory. thats what i think
thats a nice theory.. i just thought K gave it to him, so he could hide a tracking device in it and because Kumagoro was so cute, Ryuichi automatically loved it.