Most probably, the main characters would include enthusiastic trainers of different ages. There might be a budding trainer who is shy but has a very powerful Pokemon. Another could be an experienced trainer who acts as a mentor to the younger ones at the camp. And of course, there could be a mischievous trainer who always gets into trouble but has a heart of gold.
In a Pokemon summer camp fanfiction, the main characters may consist of trainers from various regions. There could be a trainer from the Kanto region who is known for his unique Pokemon - catching methods. Then there's a girl from the Johto region who has a special bond with her Eevee. Also, there could be a group of friends who have come to the camp together, each with their own dreams and goals related to Pokemon training.