Well, of course, Caillou himself is the central character. His parents also play important roles as they are the ones who ground him and teach him lessons.
Perhaps Seto got into a fight with a powerful enemy and in the process, he put others in danger. His guardians or the people in charge decide to ground him to teach him a lesson about responsibility. During his grounding, he could start to reflect on his actions and come up with new strategies to face his enemies. He might also discover new things about himself while being alone in his grounded state.
One possible plotline could be that Harry gets grounded for accidentally using magic outside of school in a Muggle area. This might lead to him having to deal with the disappointment of his friends who can't visit him as easily. Another could be that he gets grounded because he was caught sneaking out to follow a lead on a new dark wizard threat, and the adults in his life want to keep him safe by restricting his movements.
Obviously, Harry Potter himself is the central character. His friends like Ron Weasley and Hermione Granger are also often involved. They might try to visit him while he's grounded or come up with plans to get him out of being grounded. Dumbledore could also play a role, either as the one who enforces the grounding in some cases or as the one who might offer some wisdom to Harry during his time of confinement.
Maybe in the fanfiction, Caillou gets grounded for breaking his sister's favorite toy. He then spends his time grounded trying to fix it as a way to make amends, which shows his growth in taking responsibility for his actions.
Obviously, Seto himself would be the main character. He is the one who is getting grounded, so the whole story would revolve around him. His family members could also be important characters. For example, his parents or guardians who are the ones grounding him. They would play a role in setting the rules and enforcing the grounding. His friends might also be characters in the story. They could come to visit him while he is grounded or try to convince his guardians to let him go early.
The length of the story in Grounded can vary depending on how you play and explore. But typically, it offers several hours of gameplay and story content.