Since we don't know the exact nature of the 'jester novel', it's all speculation. But perhaps there's a young jester as the protagonist, and his mentor who is an older, more experienced jester. There could also be a love interest for the jester, like a beautiful maiden in the court. And of course, there might be the antagonists, perhaps a jealous nobleman who wants to get rid of the jester because he is too popular among the people.
As I haven't read the 'jester novel', I can't say for sure. But I would expect the jester to be one of the main characters. There might also be a noble or a king that the jester serves.
The main characters are, of course, the king and the jester.
In 'trapped with jester fanfiction', it might be that the main characters are the jester and a noble who has fallen from grace. They are trapped together, and the contrast between their statuses - the jester being a lower - class entertainer and the former noble - creates an interesting dynamic in the story. The noble might initially look down on the jester but as they face the challenges of being trapped, they start to see the jester in a new light.
I don't know for sure, but since it's about a jester among the league, the jester is likely a main character. There might also be other important characters within the league, such as the league's leaders, the jester's friends or rivals in the league.
The jester could interact with the main heroes in the League. For instance, they might have a lot of scenes with the most popular hero, constantly trying to make them laugh or annoy them. This would create a fun and engaging relationship between the two characters.
The jester is more interesting. He has the freedom to be creative and use humor to get his point across. He can play tricks and make witty remarks without the same level of formality as the king.
One main theme is loyalty. The jester is often in a position where he has to decide who to be loyal to. Another could be the power dynamics in the historical setting. There are those in power and those who are trying to gain power or survive under the rule of the powerful.
Yasha would be a great addition. Her relationship with Beau and Jester could add an interesting dynamic. She could be the muscle when Beau and Jester get into trouble on their adventures.
I'm not sure exactly. You might need to check the movie's credits or do some online research to find out.
Another option could be a group of pirates who believe that the Ruby of the Sea can lead them to a great treasure. They constantly try to capture the Ruby and the Jester has to fend them off to protect her. The pirates would be a constant threat throughout the fanfic.
I'm not aware of any novels specifically named 'jester novel' that are widely known. However, there are many novels that feature jesters as characters. For example, in some historical fiction novels set in medieval times, jesters often make an appearance and play important roles in the story.