1857. What character can be added to both of these characters to give other characters? 火_ -> _暴

Answer:

火  KA, hi - fire

暴  BOU, BAKU, aba(reru), aba(ku) - rage, rampage, expose

Add 火:火+火 火+暴 

火  KA, hi - fire

炎  EN, honou - flame

爆  BAKU - explode