Only 1 of 2 things can happen..
1. You equip the shinai (on yourself) and your fellow gets the stats without you seeing it, which would be lame because the 2dmg is a joke at any lvl passed 10, unless ofcourse your a mage and plan on having your npc do most of the work.
OR..
2. You simply trade the shinai to luto in upper jeuno and luto will give the weapon to your npc to use, thus giving your fellow the stats. (Which is my guess sence this is the way you upgrade ANY fellow weapon.)
I myself wont be even trying this stuff sence my fellow is equiped with a Ram-Dao, but maybe it will help others. ^^