Since I haven't read it, I can't say for sure. But I would assume the lazy king is one of the main characters. There might also be a loyal servant or an advisor who is constantly trying to get the king to be more responsible.
I don't know precisely, but probably the lazy king himself is a main character. There could be a princess or a prince who has different ideas about how the kingdom should be run compared to the lazy king. And perhaps there is a wizard or a mage who tries to use magic to either help or hinder the king's rule.
As I'm not familiar with the 'lazy king light novel', it's hard to name the exact main characters. However, based on the title, the lazy king is likely to be a central figure. There may be a group of rebels who oppose the king's lazy rule. Additionally, there could be a love interest for the king, which might play a role in motivating him to change or remain the same. Maybe there is also a wise old man in the story who serves as a mentor figure to the king or other characters, guiding them through the challenges in the kingdom.