Since it's the 'jerry the orphan boy story', Jerry is the central character. There could be a caretaker in the orphanage who is either kind or cruel to Jerry, and that person would be an important character. Also, if there are any visitors to the orphanage who interact with Jerry, they could be main characters as well.
Obviously Jerry is one of the main characters since it's his story. There might also be other orphans in the orphanage who play important roles, like his best friend there.