There could be his drill sergeant, a tough and no - nonsense character who has to whip Harry into shape. Maybe there's a fellow recruit who becomes Harry's best friend. They could support each other through the difficult training.
One possible character could be an old veteran who serves as a mentor to Harry. He could teach Harry the ropes of the military and share his own war stories. Another character might be a female soldier who Harry has a bit of a romantic interest in. She could be just as strong and capable as Harry, and they could have some interesting interactions both on and off the battlefield.