The king doesn't have to roleplay. After all, he's busy. Here's our AI.
King's advisor recommends war. King has a 30% chance of declaring war on the kingdom. ^_^ Other kingdom NPC king offers treaty approximately once a week. Receiving king NPC accepts by a role of 80%. If negative, goes to ar with kingdom if not already at war.
Simple as pie.
Besides, our shogun is the true leader anyway, the emperor is just a puppet leader.