This character archetype is a little slippery. The Soldier can sometimes be a literal soldier, like Captain America.
There are other characters that play the Soldier role without actually being in military service, either currently or previously, like Knuckles from Sonic the Hedgehog.
The pillars of the Soldier archetype are primarily a sense of loyalty, duty/honor, and usually a serious attitude. You can already see that this archetype is a little more specific than some of the others, more like the Rebel than the Paragon in that aspect.
A lot of the characters that fall into this category are strong, serious fighter-types that act as leaders in one way or another. Tygra from Thundercats, Shiro from Voltron, and Roran Stronghammer from the Inheritance Cycle, and Gale Hawthorne from the Hunger Games series would all be Soldiers, compelled to fight for what they think is right, acting as leaders when they have to, and mostly motivated by a sense of duty or loyalty to the series' Hero.
You'll notice that some of these characters are showing up more than once. How can one character fall into so many categories? Gale can't be a Rival and a Best Friend and a Soldier all at once, can he?
Well, yes. He absolutely can. But when choosing characters to fill out your story, what label you apply to that character (Chosen One, Soldier, Rival, Rebel, etc.) will describe how that character functions in relation to the rest of the cast, rather than what that character's personality is.
In this case, the Soldier is most often the protector or representative of those who are unable to protect or represent themselves. Whether or not the Soldier is nice about it, or even if they're doing the right thing, varies from story to story. The key components to the Soldier are the presence of an honor code or binding duty, in addition to a generally loyal attitude. An interesting twist on the Soldier character type is one you may or may not remember from early 2000's afternoon cartoons:
"Shego" from the cartoon Kim Possible is what I would classify as a Rebel Soldier. At one point she was a protector of the innocent and served as part of her family superhero team, "Team Go." Before the beginning of the series, she left Team Go and became a sidekick to one of the primary antagonists of the series, where she then served as a representative for the less intelligent, less efficient Dr. Drakken.
While the Soldier is most commonly a protagonist, twisting the role to serve as an antagonist can be extremely effective, carrying many of the same challenges as the Paragon and for the same reasons. Readers tend to see loyalty and honor as positive traits, but when they're shown to be amoral (like with Anton Chigurh, and his rules about the coin flip in No Country for Old Men) it can be a shock to the reader and an excellent opportunity for some knife-twisting fun.
To the previous Element
To the next Element