Emperor Go-Daigo
Go-Daigo wanted to rule Japan himself.  The bakufu government was weak because they had not been able to reward their warriors.  When the Mongols invaded, the bakufu brought an army and kept them from invading.  But the usual way to reward warriors was to give them the defeated enemy's lands and treasure.  Because the Mongols were invaders, they had no lands for the shoguns to give to their warriors.  This made the warriors unhappy with the shoguns.  Go-Daigo began to quietly raise an army to defeat the bakufu and take control of Japan, but he was found out.  The shogun caught him and imprisoned him on the Oki Islands.