1468. What character can be added to both of these characters to give other characters? 其_ -> _要

Answer:

其  shi, so - that

要  YOU, i(ru) - essential(s) need

Add 月:其+月 月+要 

月  GETSU, GATSU, tsuki - moon, month

期  KI, GO - time, period, term

腰  YOU, koshi - pelvis, loins, hips, back