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

Answer:

日  NICHI, JITSU, hi, ka - sun, day

旦  TAN, DAN, akira, tadashi, asa morning 

Add 月:日+月 月+旦 

月  GETSU, GATSU, tsuki - moon, month

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

胆  TAN - gallbladder, courage