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.
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.
It could be a new character, like a foreign exchange student from Beauxbatons. She has a different charm and magic style compared to Hogwarts students, which catches Harry's eye.
Sakura could potentially be Naruto's girlfriend in some fanfictions. Although in the original series she had feelings for Sasuke for a long time, in fan - created stories, the writers might explore a different path where she realizes Naruto's true worth and they end up together.
Perhaps an intelligent and patient psychologist. She could help Sheldon understand his emotions better. Given Sheldon's complex personality, she would need to be very understanding and be able to handle his idiosyncrasies. This type of girlfriend could also introduce Sheldon to the world of human emotions in a more systematic way.
Caillou is a children's show about a 4-year-old boy and his daily adventures and experiences. It focuses on his family, friends, and learning life lessons.
She could be a mysterious girl who has been watching Percy from afar and finally decides to approach him. Maybe she's a half - nymph with the ability to communicate with nature, which Percy finds fascinating.
Some fanfictions may explore Naruto getting a girlfriend in a completely alternate universe. Maybe in a modern - day setting where Naruto is just an ordinary high school student and he meets a girl who has no idea about his ninja past, which leads to a very different kind of relationship.