The main character could be a young trainer named Ash. He's always eager to learn more about Pokemon and is very friendly. Another character could be Misty, a girl who is really good with water - type Pokemon and is a bit of a tomboy. There could also be Brock, who is knowledgeable about Pokemon care and often gives advice to other students.
One option for a main character could be a shy girl named Lily. She has a small Pikachu as her partner Pokemon, and she gradually overcomes her shyness throughout the story as she participates in school activities with her Pikachu. Then there could be a rival character, Max. He has a strong Charmander and is always trying to outdo others in Pokemon battles. And don't forget the kind teacher, Professor Oak, who guides the students in their Pokemon - related studies.
For main characters, how about a group? There could be a trio: Tim, a boy who loves electric - type Pokemon; Amy, who is an expert in grass - type Pokemon; and Jake, who has a knack for finding and befriending wild Pokemon. They could get into all sorts of adventures together in the Pokemon school, like when a new and dangerous Pokemon shows up on the school grounds and they have to figure out how to deal with it while also learning more about Pokemon in their classes.