1691. What character can be added to both of these characters to give other characters? 日_ -> _方

Answer:

日  NICHI, JITSU, hi, ka - sun, day

方  HOU, kata - direction

Add 月:日+月 月+方 

月  GETSU, GATSU, tsuki - moon, month

明  MEI, MYOU, aka(rui), a(keru) - bright

肪  BOU - animal fat