Not having knowledge of the 'little soldier boy story' makes it impossible to accurately name the main characters. But if it follows a common pattern, the boy would be the central character. There could be other characters like his mentor if he is learning to be a soldier, or friends who support him on his journey. There might also be some characters who oppose him, like bullies or enemies in a war - related scenario.
I'm not sure specifically which 'Mako Iwamatsu Little Soldier Boy' story you are referring to. Mako Iwamatsu was a great actor, but without more context, it's difficult to tell the exact story.
I don't know the exact details of this story. It could be a unique and interesting story though, considering Mako Iwamatsu's talent in acting. Maybe it was a story that was not widely known yet.