The main character is surely the kind king. Besides, there could be some advisors around him. They might either support his kind deeds or try to persuade him to be more practical at times. And then, there might be some ordinary people in the kingdom, like farmers or artisans, whose lives are affected by the king's kindness.
In 'the kind king story', the most prominent character is the kind king himself. There may be some villains as well, who oppose the king's kind policies because they think it will make the people soft and difficult to control. But in the end, the king's kindness prevails, and those villains are defeated. There may also be some foreign envoys who are surprised by the king's kindness when they visit the kingdom.