A wise old owl could be added. It could give advice to the little white horse. Also, a mischievous fox that sometimes gets the horse into trouble but also helps it out of tight spots.
A young fairy could be a great addition. The fairy might be able to ride on the little white horse's back and together they could explore different parts of the magical realm. There could also be a big, strong bear. At first, the bear might seem intimidating, but later it becomes a friend and protector of the horse.
A young stargazer from Earth could be a new character. This character could be someone who has always been fascinated by the stars and dreams of traveling to other planets. When the Little Prince lands on Earth again, they meet and the stargazer becomes his guide on Earth, showing him all the new things that have happened since his last visit. Another potential character could be a talking moonbeam. It could offer the Little Prince advice and wisdom, as well as some magical abilities to help him on his journey.
A young, adventurous boy from a neighboring town. He could be quite different from the characters Little Miss Marker is used to. He's full of energy and new ideas, and together they could get into all sorts of escapades, like exploring an old, abandoned castle.
A mysterious old Pokemon trainer who has a lot of knowledge about ancient Pokemon moves. He could guide Rusty on his journey.
A mysterious elven archer could be added. This archer could be on a quest for revenge against the orcs and initially see Bruz as just another target. But as the story progresses, they realize that Bruz is different and they could end up working together.
An American wizard detective. He could be investigating strange magical happenings in the wizarding world of America. The Potter character could cross paths with him and end up teaming up to solve a big case.
A healer character could be added. This healer could be the one who discovers a hidden power within the apprentice that could change the course of their journey. Another potential character could be a mysterious traveler. This traveler has seen many different worlds and has knowledge that could either help or hinder the apprentice.
A young and curious wizard could be added. This wizard could be intrigued by the devil's unbreakable nature and try to study it, leading to some interesting interactions. Another character could be an angel who has lost its way and forms a strange bond with the devil. Or perhaps a talking animal, like a raven, that serves as the devil's confidant and gives it advice from a different perspective.
A scientist who is an expert on alien plants. He could be brought in to study the plant and try to find a way to control it.
One potential character could be a wise old hermit who lives in a secluded part of the paradise. He could offer Percy valuable advice and knowledge about the secrets of the paradise.
A group of nomadic mice. They have different skills and traditions compared to the Redwall mice. Their arrival at the abbey could cause some cultural clashes at first, but eventually, they could learn from each other and contribute to the abbey's growth. This would add more diversity to the characters in the fanfiction.