The main characters in the 'pokemon regi trio hall of origins fanfic' are the Regi trio. Regirock is a powerful and imposing Pokemon made of rocks. It has a strong physical presence. Regice is a cool and icy Pokemon, perhaps with a more aloof personality. And Registeel is a tough and sturdy steel - type. Their combined presence in the Hall of Origins forms the core of the story. They might interact with other Pokemon or entities within the hall, but they are the central focus.