There is Captain Edward Ashburnham, who is the 'good soldier' of the title. He appears to be a proper and noble military man on the surface.
Florence Dowell is another main character. She is married to the narrator and her actions and relationships play a significant role in the development of the story.
Obviously, the good soldier is a main character. There may also be his commanding officer who gives him orders and influences his actions.
I'm not sure specifically as I haven't read it. But it might have a protagonist who is the soldier king. There could also be his comrades, his commanders, and perhaps some antagonists like enemy soldiers or traitors within his own ranks.
In 'the winter soldier novel', the most prominent character is the Winter Soldier. However, other important characters may include those who are trying to help him break free from Hydra's control. There could be some fellow soldiers or agents who discover his true nature and decide to assist him. Also, antagonists within Hydra who will do anything to keep him under their control are likely to be part of the main characters in the story.
In the 'Soldier 76 Graphic Novel', Soldier 76 is surely a main character. Additionally, there may be characters who are important to his backstory, like his superiors or subordinates from his military career. And perhaps some new characters created specifically for this graphic novel to drive the plot forward, either by helping or hindering Soldier 76 in his journey.
In 'For a Lost Soldier' novel, perhaps there is a young boy or girl who forms a connection with the soldier. This character could bring out different sides of the soldier's personality. And of course, there may be other soldiers or military personnel who play important roles in the story, either as comrades or antagonists to the main soldier character.
One of the main themes is the destruction of illusions. The characters' false beliefs about love, loyalty, and propriety are gradually shattered throughout the story.
Perhaps there are veteran soldiers who pass on their wisdom and experiences to the new recruits in the 'Soldier Story'. These veterans with their years of service and numerous stories could be main characters.
There could be a protagonist who is a super soldier with extraordinary abilities. He or she might be leading the charge against the antagonists.
I'm not sure exactly, but it might have a strong - willed soldier as the main character.
In the 'Rune Soldier Light Novel', there might be a young and determined character who is the main focus. This character could be a novice in the world of runes at the start but gradually becomes more powerful. There may also be a mentor - like figure who guides the protagonist in understanding and using rune magic. Additionally, there could be antagonists who oppose the protagonist's growth or use of rune magic for their own nefarious purposes.