1785. What character can be added to both of these characters to give other characters? 戸_ -> _心

Answer:

戸  KO, to - door

心  SHIN, kokoro - heart

Add 非:戸+非 非+心

非  HI - mistake, non-

扉  HI, tobira - door, title page

悲  HI, kana(shii) - sad, be sad, lament