Hi Arikaa, Thanks for trying my game. In order to explore the other character routes, you have to escape from the castle during chapter 3 when the carriers are attacking.
Please check out the wiki if you need more hints. https://allele-origin.fandom.com/wiki/Allele_Origin_Wiki:%E2%99%A1
Simon:To start Simon's route, don't sit quietly and leave the closet during the castle attack in chapter 3. Follow the sound of the battle at the front gate and keep watching it unfold. When Isaac is distracted, try to make a run for it to escape the castle. The MC will later find parts of Simon. Help him and then follow him to his house.
Anthony: To start on Anthony's route, take a walk in the courtyard at the beginning of chapter 3. The MC will notice overgrown vines. Later in chapter 3, when Nate locks the MC in the closet during the attack, break free of it and choose to go back to the courtyard. Anthony will help the MC to escape the castle.
Best wishes!