"If you're not looking for responsibility, just hoping to survive, this is the job for you!"
Soldier is an unlockable helmet (i.e. specialization or class). Once acquired it grants +5 Life Points on top of the Troopers maximum, with each new level adding 1 more (i.e. level 6 (initial) will have 15 , level 7 – 16 , and so on).
A good pick for pretty much everyone, it offers a really good boost in survivability offering some degree of protection from all kinds of Damage, and even becoming able to soak some damage due to its effect wearing off before the trooper can die (5~11 ). It starts off as a pretty unremarkable bonus (some blue Skills give a greater increase in LP than the initial 5), but a Soldier that keeps gaining levels will soon become able to outlive most troopers. Paired with Bait it effectively makes the trooper a living shield able to take many shots that would otherwise kill his allies.
There's one very effective counter – Killshots.
A Soldier can reduce the chances of being on the receiving end of one with the Full Metal Balaclava (which paired with the helmet negates one headshot and halves the chance of subsequent ones) and Bulletproof Vest which helps protect from torso killshots. If the trooper can acquire Invincible, though, all the worries are gone for good.
Another weak point of a Soldier is that he can accumulate body part damage and thus receive huge amounts of damage from already-hit body parts unless a Doctor can intervene quickly: Hard Boiled is very useful in protecting against this, as well as negating Aim and Speed penalties, and thus keep the Soldier offensively efficient no matter the wounds.
Also, short-ranged soldiers can be stopped in their tracks with Paralysing Shells. As long as they're firing away, long-ranged weaklings under your command should be relatively fine.
Abilities such as Brick S***house and Bait make for an outstanding combination, letting the Soldier draw and dampen most of the enemy fire; the Life Point bonus added by them is the icing on the cake.
For offensive soldiers, Charge is noticeable in greater damage dealt compared to other classes.